MR. JUSTICE MARSHALL delivered the opinion of the Court.[fn*] [fn*]56MR. JUSTICE WHITE joins Parts I, II-A, and III of this opinion.

The issue presented is whether the United States may appeal in a criminal case from a midtrial ruling resulting in the exclusion of certain evidence and from a subsequently entered judgment of acquittal. Resolution of this issue depends on the application of the Double Jeopardy Clause of the Fifth Amendment to the somewhat unusual facts of this case.

I

Petitioner was indicted, along with several others, for violating 18 U.S.C. § 1955 (1976 ed.), which makes it a federal offense to conduct, finance, manage, supervise, direct, or own all or part of an "illegal gambling business." § 1955(a). Such a business is defined as one that is conducted by five or more persons in violation of the law of the place where the business is located and that operates for at least 30 days or earns at least $2,000 in any one day. § 1955(b)(1). The single-count indictment here charged in relevant part that the defendants' gambling business involved "accepting, recording and registering bets and wagers on a parimutual [ sic] number pool and on the result of a trial and contest of skill, speed, and endurance of beast," and that the business "was a violation of the laws of the Commonwealth of Massachusetts, to wit, M.G.L.A. Chapter 271, Section 17."

The Government's evidence at trial showed the defendants to have been engaged primarily in horse betting and numbers betting. At the close of the Government's case, petitioner's counsel, who represented 8 of the 11 defendants, moved for a judgment of acquittal as to all of his clients. Joined by counsel for other defendants, he argued, inter alia, that the Government had failed to prove that there was a violation of the state statutory section as alleged in the indictment, since Mass. Gen. Laws Ann., ch. 271, § 17 (West 1970), as construed by the state courts, did not prohibit numbers betting but applied only to betting on "games of competition" such as horse races. The Government responded that "violation of the State law is a jurisdictional element of [the federal] statute" and that "not every [defendant] must be found to be violating this State law." The District Court accepted the Government's theory and denied the defendants' motion, stating that "a defendant to be convicted must [only] be found to have joined in [the illegal] enterprise in some way."

Petitioner's counsel then sought clarification of whether "the numbers pool allegation [was] still in the case." The court indicated that it was, because counsel had not presented any state-court authority for the proposition that § 17 did not include numbers betting. The court also expressed the view, however, that if petitioner's counsel were correct, "we would have to exclude . . . all of the evidence that has to do with bets o[n] numbers." The Government demurred, arguing that exclusion of the numbers evidence would "not necessarily follow" from acceptance of petitioner's theory. Taking his lead from the court, petitioner's counsel next moved "to strike or limit the evidence." The motion was denied.

After the defendants had rested, the trial judge announced that he was reversing his earlier ruling on the motion to exclude evidence, because he had discovered a Massachusetts case holding that numbers betting was not prohibited by § 17, but only by § 7 of ch. 271. The court then struck all evidence of numbers betting, apparently because it believed such action to be required by the indictment's failure to set forth the proper section.

At this point counsel moved for a judgment of acquittal as to petitioner alone, arguing that there was no evidence of his connection with horse-betting activities. The Government did not disagree that the evidence was insufficient to show petitioner's involvement with a horse-betting operation, but repeated its earlier argument relating to the "jurisdictional" nature of the state-law violation. The court rejected this contention, stating that the offense had "to be established in the terms that you [the Government] charged it, which was as a violation of § 17" and that petitioner had to be "connected with this operation, and by that I mean a horse operation." The court concluded: "I don't think you've done it." It then granted petitioner's motion for a judgment of acquittal and entered an order embodying this ruling later that day.

The next day the Government moved the court to reconsider both "its ruling . . . striking . . . evidence concerning the operation of an illegal . . . numbers pool" and "its decision granting defendant Thomas Sanabria's motion for judgement [ sic] of acquittal." Prompted by the Government's arguments in support of reconsideration, the court asked defense counsel why he had not raised the objection to the indictment's citation of § 17 earlier and what prejudice resulted to petitioner from the failure to cite the proper section. Counsel responded that the objection had not "ripened" until, at the end of the Government's case, the court was asked to take judicial notice of § 17, and that he need not and did not allege actual prejudice. The court denied the motions to reconsider, but indicated that, had it granted the motion to restore the numbers evidence, it also would have vacated the judgment of acquittal. The case against the remaining 10 defendants went to the jury on a theory that the gambling business was engaged in horse betting; all were convicted.

The Government filed a timely appeal "from [the] decision and order . . . excluding evidence and entering a judgment of acquittal . . . and . . . denying the Motion for Reconsideration." Conceding that there could be no review of the District Court's ruling that there was insufficient evidence of petitioner's involvement with horse betting, the Government sought a new trial on the portion of the indictment relating to numbers betting.

The Court of Appeals for the First Circuit held first that it had jurisdiction of the appeal. Although the jurisdictional statute, 18 U.S.C. § 3731 (1976 ed.), by its terms authorizes the Government to appeal only from orders "dismissing an indictment . . . as to any one or more counts." the word "count" was "interpret[ed] . . . to refer to any discrete basis for the imposition of criminal liability." 548 F.2d 1, 5 (1976). Viewing the horse-betting and numbers allegations as "discrete bas[es] of criminal liability" duplicitously joined in a single count, the court characterized the District Court's action as a "dismissal" of the numbers "charge" and an acquittal for insufficient evidence on the horse-betting charge. Id., at 4-5, and n. 4. It concluded that § 3731 authorized an appeal from the "dismissal" of the numbers charge, "if the double jeopardy clause does not bar a future prosecution on this charge." 548 F.2d, at 5.

Consistent with its above analysis, the court found that petitioner had voluntarily terminated the proceedings on the numbers portion of the count by moving, in effect, to dismiss it. Since the "dismissal" imported no ruling on petitioner's "criminal liability as such," and since petitioner's motion was not attributable to "prosecutorial or judicial overreaching," the court applied the rule permitting retrials after a prosecution is terminated by a defendant's request for a mistrial. Id., at 7-8, citing United States v. Dinitz, 424 U.S. 600 (1976). There being no double jeopardy bar to a new trial, the court went on to resolve the merits of the appeal in the Government's favor. It held, based on an intervening First Circuit decision, that the District Court had erred in "dismissing" the numbers theory. Accordingly, the judgment of acquittal was "vacated" and the case "remanded so that the government may try defendant on that portion of the indictment that charges a violation of § 1955 based upon numbering [ sic] activities." 548 F.2d, at 8.

We granted certiorari, 433 U.S. 907 (1977), limiting our review to the related issues of appealability and double jeopardy. We now reverse.

II

In United States v. Wilson, 420 U.S. 332 (1975), we found that the primary purpose of the Double Jeopardy Clause was to prevent successive trials, and not Government appeals per se. Thus we held that, where an indictment is dismissed after a guilty verdict is rendered, the Double Jeopardy Clause did not bar an appeal since the verdict could simply be reinstated without a new trial if the Government were successful. That a new trial will follow upon a Government appeal does not necessarily forbid it, however, because in limited circumstances a second trial on the same offense is constitutionally permissible. Appealability in this case therefore turns on whether the new trial ordered by the court below would violate the command of the Fifth Amendment that no "person [shall] be subject for the same offence to be twice put in jeopardy of life or limb."

In deciding whether a second trial is permissible here, we must immediately confront the fact that petitioner was acquitted on the indictment. That "'[a] verdict of acquittal . . . [may] not be reviewed . . . without putting [the defendant] twice in jeopardy, and thereby violating the Constitution,'" has recently been described as "the most fundamental rule in the history of double jeopardy jurisprudence." United States v. Martin Linen Supply Co., 430 U.S. 564, 571 (1977), quoting United States v. Ball, 163 U.S. 662, 671 (1896). The fundamental nature of this rule is manifested by its explicit extension to situations where an acquittal is "based upon an egregiously erroneous foundation." Fong Foo v. United States, 369 U.S. 141, 143 (1962); see Green v. United States, 355 U.S. 184, 188 (1957). In Fong Foo the Court of Appeals held that the District Court had erred in various rulings and lacked power to direct a verdict of acquittal before the Government rested its case. We accepted the Court of Appeals' holding that the District Court had erred, but nevertheless found that the Double Jeopardy Clause was "violated when the Court of Appeals set aside the judgment of acquittal and directed that petitioners be tried again for the same offense." 369 U.S., at 143. Thus when a defendant has been acquitted at trial he may not be retried on the same offense, even if the legal rulings underlying the acquittal were erroneous.

The Government does not take issue with these basic principles. Indeed, it concedes that the acquittal for insufficient evidence on what it refers to as the horse-betting theory of liability is unreviewable and bars a second trial on that charge. The disputed question, however, is whether a retrial on the numbers theory of liability would be on the "same offense" as that on which petitioner has been acquitted.

The Government contends, in accordance with the reasoning of the Court of Appeals, that the numbers theory was dismissed from the count before the judgment of acquittal was entered and therefore that petitioner was not acquitted of the numbers theory. Petitioner responds that the District Court did not "dismiss" anything but rather struck evidence and acquitted petitioner on the entire count; further, assuming arguendo that there was a "dismissal" of the numbers theory, he urges that a retrial on this theory would nevertheless be barred as a second trial on the same statutory offense. We first consider whether the Court of Appeals correctly characterized the District Court's action as a "dismissal" of the numbers theory.

A

In the Government's view, the numbers theory was "dismissed" from the case as effectively as if the Government had actually charged the crime in two counts and the District Court had dismissed the numbers count. The first difficulty this argument encounters is that the Government did not in fact charge this offense in two counts. Legal consequences ordinarily flow from what has actually happened, not from what a party might have done from the vantage of hindsight. See Central Tablet Mfg. Co. v. United States, 417 U.S. 673, 690 (1974). The precise manner in which an indictment is drawn cannot be ignored, because an important function of the indictment is to ensure that, "in case any other proceedings are taken against [the defendant] for a similar offence, . . . the record [will] sho[w] with accuracy to what extent he may plead a former acquittal or conviction." Cochran v. United States, 157 U.S. 286, 290 (1895), quoted with approval in Russell v. United States, 369 U.S. 749, 764 (1962); Hanger v. United States, 285 U.S. 427, 431 (1932).

With regard to the one count that was in fact charged, as to which petitioner has been at least formally acquitted, we are not persuaded that it is correct to characterize the trial court's action as a "dismissal" of a discrete portion of the count. While form is not to be exalted over substance in determining the double jeopardy consequences of a ruling terminating a prosecution, Serfass v. United States, 420 U.S. 377, 392-393 (1975); United States v. Jorn, 400 U.S. 470, 478 n. 7 (1971); United States v. Goldman, 277 U.S. 229, 236 (1928), neither is it appropriate entirely to ignore the form of order entered by the trial court, see United States v. Barber, 219 U.S. 72, 78 (1911). Here the District Court issued only two orders, one excluding certain evidence and the other entering a judgment of acquittal on the single count charged. No language in the indictment was ordered to be stricken, compare United States v. Alberti, 568 F.2d 617, 621 (CA2 1977), nor was the indictment amended. The judgment of acquittal was entered on the entire count and found petitioner not guilty of the crime of violating 18 U.S.C. § 1955 (1976 ed.), without specifying that it did so only with respect to one theory of liability:

"The defendant having been set to the bar to be tried for the offense of unlawfully engaging in an illegal gambling business, in violation of Title 18, United States Code, Sections 1955 and 2, and the Court having allowed defendant's motion for judgment of acquittal at the close of government's evidence,

"It is hereby ORDERED that the defendant Thomas Sanabria be, and he hereby is, acquitted of the offense charged, and it is further ORDERED that the defendant Thomas Sanabria is hereby discharged to go without day."

The Government itself characterized the District Court's ruling from which it sought to appeal as "a decision and order . . . excluding evidence and entering a judgment of acquittal." Notice of Appeal. Similar language appears in its motion for reconsideration filed in the District Court. Indeed, the view that the trial court "dismissed" as to one "discrete basis of liability" appears to have originated in the opinion below. Thus, not only defense counsel and the trial court but the Government as well seemed in agreement that the trial court had made an evidentiary ruling based on its interpretation of the indictment.

We must assume that the trial court's interpretation of the indictment was erroneous. See n. 13, supra. But not every erroneous interpretation of an indictment for purposes of deciding what evidence is admissible can be regarded as a "dismissal." Here the District Court did not find that the count failed to charge a necessary element of the offense, cf. Lee v. United States, 432 U.S. 23 (1977); rather, it found the indictment's description of the offense too narrow to warrant the admission of certain evidence. To this extent, we believe the ruling below is properly to be characterized as an erroneous evidentiary ruling, which led to an acquittal for insufficient evidence. That judgment of acquittal, however erroneous, bars further prosecution on any aspect of the count and hence bars appellate review of the trial court's error. United States v. Martin Linen Supply Co., 430 U.S., at 571; Fong Foo v. United States, 369 U.S. 141 (1962); Green v. United States, 355 U.S., at 188; United States v. Ball, 163 U.S., at 671.

B

Even if the Government were correct that the District Court "dismissed" the numbers allegation, in our view a retrial on that theory would subject petitioner to a second trial on the "same offense" of which he has been acquitted.

It is Congress, and not the prosecution, which establishes and defines offenses. Few, if any, limitations are imposed by the Double Jeopardy Clause on the legislative power to define offenses. Brown v. Ohio, 432 U.S. 161, 165 (1977). But once Congress has defined a statutory offense by its prescription of the "allowable unit of prosecution," United States v. Universal C. I. T. Credit Corp., 344 U.S. 218, 221 (1952); Bell v. United States, 349 U.S. 81 (1955); Braverman v. United States, 317 U.S. 49 (1942); In re Nielsen, 131 U.S. 176 (1889), that prescription determines the scope of protection afforded by a prior conviction or acquittal. Whether a particular course of conduct involves one or more distinct "offenses" under the statute depends on this congressional choice.

The allowable unit of prosecution under § 1955 is defined as participation in a single "illegal gambling business." Congress did not assimilate state gambling laws per se into the federal penal code, nor did it define discrete acts of gambling as independent federal offenses. See H.R. Rep. No. 91-1549, p. 53 (1970). See also Iannelli v. United States, 420 U.S. 770, 784-790 (1975). The Government need not prove that the defendant himself performed any act of gambling prohibited by state law. It is participation in the gambling business that is a federal offense, and it is only the gambling business that must violate state law. And, as the Government recognizes, under § 1955 participation in a single gambling business is but a single offense, "no matter how many state statutes the enterprise violated." Brief for United States 31.

The Government's undisputed theory of this case is that there was a single gambling business, which engaged in both horse betting and numbers betting. With regard to this single business, participation in which is concededly only a single offense, we have no doubt that petitioner was truly acquitted.

We have recently defined an acquittal as "'a resolution, correct or not, of some or all of the factual elements of the offense charged.'" Lee v. United States, 432 U.S., at 30 n. 8, quoting United States v. Martin Linen Supply Co., supra, at 571. Petitioner was found not guilty for a failure of proof on a key "factual element of the offense charged": that he was "connected with" the illegal gambling business. See supra, at 59. Had the Government charged only that the business was engaged in horse betting and had petitioner been acquitted, his acquittal would bar any further prosecution for participating in the same gambling business during the same time period on a numbers theory. That the trial court disregarded the Government's allegation of numbers betting does not render its acquittal on the horse-betting theory any less an acquittal on the "offense" charged. "The Double Jeopardy Clause is not such a fragile guarantee that . . . its limitations [can be avoided] by the simple expedient of dividing a single crime into a series of temporal or spatial units," Brown v. Ohio, 432 U.S., at 169, or, as we hold today, into "discrete bases of liability" not defined as such by the legislature. See id., at 169 n. 8.

While recognizing that only a single violation of the statute is alleged under either theory, the Government nevertheless contends that separate counts would have been proper, and that an acquittal of petitioner on a horse-betting count would not bar another prosecution on a numbers count. Brief for United States 33. Although there may be circumstances in which this is true, petitioner here was acquitted for insufficient proof of an element of the crime which both such counts would share — that he was "connected with" the single gambling business. See supra, at 59. This finding of fact stands as an absolute bar to any further prosecution for participation in that business.

The Government having charged only a single gambling business, the discrete violations of state law which that business may have committed are not severable in order to avoid the Double Jeopardy Clause's bar on retrials for the "same offense." Indeed, the Government's argument that these are discrete bases of liability warranting reprosecution following a final judgment of acquittal on one such "discrete basis" is quite similar to an unsuccessful argument that it presented in Braverman v. United States, 317 U.S. 49 (1942). Braverman had been convicted of and received consecutive sentences on four separate counts of conspiracy, each count alleging a conspiracy to violate a separate substantive provision of the federal narcotics laws. The Government conceded that only a single conspiracy existed, as it concedes here that only a single gambling business existed; nonetheless, it urged that separate punishments were appropriate because the single conspiracy had several discrete objects. We firmly rejected that argument:

"[T]he precise nature and extent of the conspiracy must be determined by reference to the agreement which embraces and defines its objects. Whether the object of a single agreement is to commit one or many crimes, it is in either case that agreement which constitutes the conspiracy which the statute punishes. The one agreement cannot be taken to be several agreements and hence several conspiracies because it envisages the violation of several statutes rather than one." Id., at 53.

The same reasoning must also apply where the essence of the crime created by Congress is participation in a "business," rather than participation in an "agreement."

The Double Jeopardy Clause is no less offended because the Government here seeks to try petitioner twice for this single offense, instead of seeking to punish him twice as it did in Braverman. "If two offenses are the same . . . for purposes of barring consecutive sentences at a single trial, they necessarily will be the same for purposes of barring successive prosecutions." Brown v. Ohio, supra, at 166. Accordingly, even if the numbers allegation were "dismissed," we conclude that a subsequent trial of petitioner for conducting the same illegal gambling business as that at issue in the first trial would subject him to a second trial on the "same offense" of which he was acquitted.

III

The only question remaining is whether any of the exceptions to the constitutional rule forbidding successive trials on the same offense, see n. 15, supra, apply here. The short answer to this question is that there is no exception permitting retrial once the defendant has been acquitted, no matter how "egregiously erroneous," Fong Foo v. United States, 369 U.S., at 143, the legal rulings leading to that judgment might be. The Government nevertheless argues, relying principally on Lee v. United States, 432 U.S. 23 (1977), and Jeffers v. United States, 432 U.S. 137 (1977), that petitioner waived his double jeopardy rights by moving to "dismiss" the numbers allegation and by not objecting to the form of the allegation prior to trial.

In Lee we held a retrial permissible because the District Court's midtrial decision granting the defendant's motion to dismiss the indictment for failure to state an offense was "functionally indistinguishable from a declaration of mistrial" at the defendant's request. 432 U.S., at 31. The mistrial analogy relied on in Lee is manifestly inapposite here. Although jeopardy had attached in Lee, no verdict had been rendered; indeed, petitioner conceded that "the District Court's termination of the first trial was not an acquittal," id., at 30 n. 8. Here, by contrast, the trial proceeded to verdict, and petitioner was acquitted. While in Lee the trial court clearly did contemplate a reprosecution when it granted defendant's motion, id., at 30-31, neither petitioner's motion here nor the trial court's rulings contemplated a second trial — nor could they have, since only a single offense was involved and petitioner went to judgment on that offense. Where a trial terminates with a judgment of acquittal, as here, "double jeopardy principles governing the permissibility of retrial after a declaration of mistrial," Lee v. United States, 432 U.S., at 31, have no bearing.

Nor does Jeffers support the Government's position. The defendant there was first tried and convicted of conspiring to distribute narcotics in violation of 21 U.S.C. § 846. Eight Members of the Court agreed that his subsequent trial for conducting a continuing criminal enterprise in violation of 21 U.S.C. § 848 during the same time period was on the "same offense," since the § 846 violation was a lesser included offense to the § 848 violation. Prior to the first trial, however, Jeffers had specifically opposed the Government's effort to try both indictments together, in part on the ground that they involved distinct offenses. 432 U.S., at 144 n. 8. Reasoning that Jeffers necessarily contemplated a second trial, four Members of the Court found that he had "elect[ed] to have the two offenses tried separately," id., at 152, and, by not raising the potential double jeopardy problem, had waived any objection on that ground to successive trials, id., at 152-154. The instant case presents quite a different situation. Petitioner's counsel never argued that horse betting and numbers were distinct offenses, a fortiori did not argue for or contemplate separate trials on each theory, and a multo fortiori did not "elect" to undergo successive trials.

Finally, we agree with the Court of Appeals that this case does not present the hypothetical situation on which we reserved judgment in Serfass v. United States, of "'a defendant who is afforded an opportunity to obtain a determination of a legal defense prior to trial and nevertheless knowingly allows himself to be placed in jeopardy before raising the defense.'" 420 U.S., at 394, quoting Solicitor General; see 548 F.2d, at 7. Petitioner did not have a "legal defense" to the single offense charged: participating in an illegal gambling business in violation of § 1955. Unlike questions of whether an indictment states an offense, a statute is unconstitutional, or conduct set forth in an indictment violates the statute, what proof may be presented in support of a valid indictment and the sufficiency of that proof are not "legal defenses" required to be or even capable of being resolved before trial. In all of the former instances, a ruling in the defendant's favor completely precludes conviction, at least on that indictment. Here, even if the numbers language had been struck before trial, there was no "legal" reason why petitioner could not have been convicted on this indictment, as were his 10 codefendants. The acquittal resulted from the insufficiency of the Government's proof at trial to establish petitioner's connection with the gambling business, as the trial judge erroneously understood it to have been charged.

The Government's real quarrel is with the judgment of acquittal. While the numbers evidence was erroneously excluded, the judgment of acquittal produced thereby is final and unreviewable. Neither 18 U.S.C. § 3731 (1976 ed.) nor the Double Jeopardy Clause permits the Government to obtain relief from all of the adverse rulings — most of which result from defense motions — that lead to the termination of a criminal trial in the defendant's favor. See United States v. Wilson, 420 U.S., at 351-352; S. Rep. No. 91-1296, p. 2 (1970). To hold that a defendant waives his double jeopardy protection whenever a trial court error in his favor on a midtrial motion leads to an acquittal would undercut the adversary assumption on which our system of criminal justice rests, see Jeffers v. United States, 432 U.S., at 159-160 (STEVENS, J., dissenting in part and concurring in judgment in part), and would vitiate one of the fundamental rights established by the Fifth Amendment.

The trial court's rulings here led to an erroneous resolution in the defendant's favor on the merits of the charge. As Fong Foo v. United States makes clear, the Double Jeopardy Clause absolutely bars a second trial in such circumstances. The Court of Appeals thus lacked jurisdiction of the Government's appeal.

Accordingly, the judgment of the Court of Appeals is

Reversed.