Justice Rehnquist
delivered the opinion of the Court.
In March 1980, when the Environmental Protection Agency (EPA) tried to inspect one of respondent Stauffer Chemical Co.’s Tennessee plants using private contractors in addition to full-time EPA employees, Stauffer refused to allow the private contractors to enter the plant. Stauffer argues that private contractors are not “authorized representatives” as that term is used in § 114(a)(2) of the Clean Air Act, 84 Stat. 1687, 42 U. S. C. § 7414(a)(2) (1976 ed., Supp. V). Stauffer also argues that the Government should be estopped from relitigating the question of whether private contractors are “authorized representatives” under the statute because it has already litigated that question against Stauffer and lost in connection with an attempted inspection of one of Stauffer’s plants in Wyoming. The Court of Appeals agreed with Stauffer on the merits and also on the collateral-estoppel issue. Without reaching the merits, we affirm the Court of Appeals’ holding that the Government is estopped from relitigating the statutory issue against Stauffer.
On March 27, 1980, officials from EPA and the State of Tennessee, accompanied by employees of a private firm under contract to EPA, attempted to inspect Stauffer’s elemental phosphorus production plant in Mt. Pleasant, Tenn. Stauffer refused entry to the private contractors unless they would sign an agreement not to disclose trade secrets. When the private contractors refused to do so, the entire group left without making the inspection. EPA later obtained an administrative warrant authorizing the private employees to conduct the inspection, and Stauffer refused to honor the warrant.
On the following day, EPA began a civil contempt proceeding against Stauffer in Federal District Court in Tennessee, and Stauffer simultaneously moved to quash the warrant. It argued that private contractors are not “authorized representatives” under § 114(a)(2) of the Clean Air Act for the purposes of conducting inspections of premises subject to regulation under that Act. The District Court denied Stauffer’s motion to quash, accepting EPA’s argument that the inspection authority conferred upon “authorized representatives” by the statute extends to private contractors retained by EPA. 511 F. Supp. 744 (MD Tenn. 1981).
On appeal, Stauffer reiterated its statutory argument and also asserted that the Government should be collaterally estopped on the basis of the decision in Stauffer Chemical Co. v. EPA, 647 F. 2d 1075 (CA10 1981) (hereinafter Stauffer I), from contending that § 114(a)(2) authorizes private contractors to conduct inspections of Stauffer’s plants. In Stauffer I officials of EPA and the State of Wyoming, accompanied by employees of a different private firm under contract to EPA, attempted to conduct an inspection of Stauffer’s phosphate ore processing plant near Sage, Wyo. As in the present case, Stauffer insisted that the private contractors sign a nondisclosure agreement, and when they declined to do so, Stauffer refused to allow them to enter the plant. EPA obtained an administrative warrant authorizing the private contractors to conduct the inspection, and Stauffer refused to honor the warrant. Stauffer then instituted an action in United States District Court in Wyoming seeking to quash the warrant and to enjoin EPA from using private contractors in inspecting Stauffer’s Wyoming plants. The District Court issued the injunction, and the United States Court of Appeals for the Tenth Circuit affirmed, holding that private contractors are not “authorized representatives” pursuant to § 114(a)(2). Id., at 1079.
The Sixth Circuit in the present case (hereinafter Stauffer II) reversed the District Court, adopting alternative grounds for its decision. Judge Weick, who delivered the opinion of the court, agreed with the Tenth Circuit that private contractors are not authorized to conduct inspections under the Clean Air Act. 684 F. 2d 1174, 1181-1190 (1982). Relying on Montana v. United States, 440 U. S. 147 (1979), he also held that the Government was collaterally estopped by Stauffer I from litigating the statutory question again against Stauffer. 684 F. 2d, at 1179-1181. Judge Jones wrote a separate opinion concurring on the collateral-estoppel issue and concluding that it was inappropriate for the court to reach the merits. Id., at 1190-1192. Judge Siler also wrote separately, dissenting from Judge Weick’s opinion on the collateral-estoppel issue but concurring in his opinion on the merits. Id., at 1192-1193. For the reasons which follow, we agree that the doctrine of mutual defensive collateral estoppel is applicable against the Government to preclude relitigation of the same issue already litigated against the same party in another case involving virtually identical facts. Accordingly, we affirm the judgment of the Court of Appeals without reaching the merits.
In Montana v. United States, supra, we held that the United States was estopped from relitigating in federal court the question of whether the Montana gross receipts tax on contractors of public, but not private, construction firms violates the Supremacy Clause of the United States Constitution. A public contractor, financed and directed by the Federal Government, had already litigated that question in state court, and the Montana Supreme Court unanimously had upheld the tax. In approving the defensive use of collateral estoppel against the Government in Montana, we first determined that there was mutuality of parties, see United States v. Mendoza, ante, at 164, n. 9, that the issue sought to be relitigated was identical to the issue already unsuccessfully litigated in state court, and that there had been no change in controlling facts or legal principles since the state-court action. 440 U. S., at 155-162.
We next looked to see whether there were any special circumstances warranting an exception to the otherwise applicable rules of preclusion. One exception which we mentioned as possibly relevant is the exception for “unmixed questions of law” arising in “successive actions involving unrelated subject matter.” Id., at 162; see United States v. Moser, 266 U. S. 236, 242 (1924). Noting that the exception first articulated in Moser is “difficult to delineate,” 440 U. S., at 163, we nonetheless had no trouble finding it inapplicable in Montana because of the close alignment in both time and subject matter between the federal-court and the state-court actions. Ibid. ,
Like Montana the case at bar involves the defensive use of collateral estoppel against the Government by a party to a prior action. The Government does not argue that the § 114(a)(2) issues in Stauffer I and Stauffer II are dissimilar nor that controlling law or facts have changed since Stauffer I. The Government instead argues that an exception to the normal rules of estoppel should apply because the statutory question here is an “unmixed question of law” arising in substantially unrelated actions. It also argues that the special role of the Government in litigating recurring issues of public importance warrants an exception in cases such as this one. We disagree with both of the Government’s arguments.
As commonly explained, the doctrine of collateral estoppel can apply to preclude relitigation of both issues of law and issues of fact if those issues were conclusively determined in a prior action. United States v. Mendoza, ante, p. 154; Allen v. McCurry, 449 U. S. 90, 94 (1980). Our cases, however, recognize an exception to the applicability of the principles of collateral estoppel for “unmixed questions of law” arising in “successive actions involving unrelated subject matter.” Montana v. United States, supra, at 162; see also Allen v. McCurry, supra, at 95, n. 7; United States v. Moser, supra, at 242. While our discussion in Montana indicates that the exception is generally recognized, we are frank to admit uncertainty as to its application. The exception seems to require a determination as to whether an “issue of fact” or an “issue of law” is sought to be relitigated and then a determination as to whether the “issue of law” arises in a successive case that is so unrelated to the prior case that relitigation of the issue is warranted. Yet we agree that, for the purpose of determining when to apply an estoppel,
“[w]hen the claims in two separate actions between the same parties are the same or are closely related ... it is not ordinarily necessary to characterize an issue as one of fact or of law for purposes of issue preclusion. ... In such a case, it is unfair to the winning party and an unnecessary burden on the courts to allow repeated litigation of the same issue in what is essentially the same controversy, even if the issue is regarded as one of ‘law’.” Restatement (Second) of Judgments §28, Comment b (1982).
Thus in Montana, without assigning the label “issue of law” to the claim sought to be relitigated, we determined that the exception was inapplicable because of the close alignment of time and subject matter between the state-court action and the federal-court action. If the exception was inapplicable in Montana, as we held that it was, we have no trouble concluding that it is also inapplicable here.
Both Stauffer I and Stauffer II arose as a result of EPA’s overview inspection program for supervising state efforts to enforce national air quality standards. See n. 1, supra. In both cases private contractors, in addition to EPA and state employees, tried to inspect plants owned by respondent. The inspections occurred just over two weeks apart, and in each case, Stauffer refused to allow the private contractors to enter its plant. Any factual differences between the two cases, such as the difference in the location of the plants and the difference in the private contracting firms involved, are of no legal significance whatever in resolving the issue presented in both cases.
Admittedly the purpose underlying the exception for “unmixed questions of law” in successive actions on unrelated claims is far from clear. But whatever its purpose or extent, we think that there is no reason to apply it here to allow the Government to litigate twice with the same party an issue arising in both cases from virtually identical facts. Indeed we think that applying an exception to the doctrine of mutual defensive estoppel in this case would substantially frustrate the doctrine’s purpose of protecting litigants from burdensome relitigation and of promoting judicial economy. See Parklane Hosiery Co. v. Shore, 439 U. S. 322, 326 (1979).
The Government attempts unpersuasively to supply justifications for overriding those economy concerns and allowing relitigation in cases such as this one. It argues here, as it did in United States v. Mendoza, ante, p. 154, that the application of collateral estoppel in Government litigation involving recurring issues of public importance will freeze the development of the law. But we concluded in United States v. Mendoza that that argument is persuasive only to prevent the application of collateral estoppel against the Government in the absence of mutuality. When estoppel is applied in a case where the Government is litigating the same issue arising under virtually identical facts against the same party, as here, the Government’s argument loses its force. The Sixth Circuit’s decision prevents EPA from relitigating the § 114(a)(2) issue with Stauffer, but it still leaves EPA free to litigate the same issue in the future with other litigants.
The Government also argues that because EPA is a federal agency charged with administering a body of law nationwide, the application of collateral estoppel against it will require EPA to apply different rules to similarly situated parties, thus resulting in an inequitable administration of the law. For example, EPA points to the situation created by the recent decision in Bunker Hill Company Lead & Zinc Smelter v. EPA, 658 F. 2d 1280 (1981), where the Ninth Circuit accepted EPA’s argument that § 114(a)(2) authorizes inspections by private contractors. EPA argues that if it is foreclosed from relitigating the statutory issue with Stauffer, then Stauffer plants within the Ninth Circuit will benefit from a rule precluding inspections by private contractors while plants of Stauffer’s competitors will be subject to the Ninth Circuit’s contrary rule. Tr. of Oral Arg. 17-18. Whatever the merits of EPA’s argument, for the purpose of deciding this case, it is enough to say that the issue of whether EPA would be estopped in the Ninth Circuit is not before the Court. Following our usual practice of deciding no more than is necessary to dispose of the case before us, we express no opinion on that application of collateral estoppel.
We therefore find the Government’s arguments unpersuasive in this case as justifications for limiting otherwise applicable rules of estoppel. Because we conclude that the Court of Appeals was correct in applying the doctrine of collateral estoppel against the Government here, we decline to reach the merits of the statutory question in this case. See Montana v. United States, 440 U. S., at 153. On the estoppel issue, therefore, the judgment of the Court of Appeals is
Affirmed.
To carry out its role under the Clean Air Act of supervising the States in their enforcement of national air quality standards, see 84 Stat. 1678, 1680, 1685, 42 U. S. C. §§ 7407, 7410, 7412 (1976 ed., Supp. V), EPA annually inspects approximately 10% of the major stationary sources of air pollution within each State. See Brief for United States 1, n. 2. Section 114(a)(2) provides that “the Administrator or his authorized representative, upon presentation of his credentials . . . shall have a right of entry” to conduct such inspections. 42 U. S. C. § 7414(a)(2) (1976 ed., Supp. V).
Stauffer raised its estoppel argument for the first time in the Court of Appeals. It did not argue to the District Court in Tennessee that EPA should be estopped by the prior decision of the Wyoming District Court in Stauffer I. Although the Wyoming District Court had decided Stauffer I by the time the Tennessee District Court decided this case, it had relied on alternative grounds for its decision. See In re Stauffer Chemical Co., 14 ERC 1737 (1980). By the time this case reached the Sixth Circuit, however, the Tenth Circuit had affirmed the District Court in Stauffer I solely on the ground that § 114(a)(2) does not authorize inspections by private contractors.
The description of the exception in United States v. Moser is not very illuminating. There we stated:
“[Estoppel] does not apply to unmixed questions of law. Where, for example, a court in deciding a case has enunciated a rule of law, the parties in a subsequent action upon a different demand are not estopped from insisting that the law is otherwise, merely because the parties are the same in both cases. But a fact, question or right distinctly adjudged in the original action cannot be disputed in a subsequent action, even though the determination was reached upon an erroneous view or by an erroneous application of the law.” 266 U. S., at 242 (emphasis in original).
In Montana we paraphrased the exception as applying to “issues of law [which] arise in successive actions involving unrelated subject matter.” 440 U. S., at 162.
An exception which requires a rigid determination of whether an issue is one of fact, law, or mixed fact and law, as a practical matter, would often be impossible to apply because “the journey from a pure question of fact to a pure question of law is one of subtle gradations rather than one marked by a rigid divide.” Restatement (Second) of Judgments § 28, Comment b (1982).
The Government argues for a broader interpretation of the exception. Relying on Moser1 s language that parties are not estopped in a “subsequent action upon a different demand,” United States v. Moser, 266 U. S., at 242, the Government argues that two cases must have more in common than the same parties and the same legal issue to constitute the same “demand” for estoppel purposes. Thus the Government’s argument essentially is that two cases presenting the same legal issue must arise from the very same facts or transaction before an estoppel can be applied. Whatever applicability that interpretation may have in the tax context, see Commissioner v. Sunnen, 333 U. S. 591, 601-602 (1948) (refusing to apply an estoppel when two tax cases presenting the same issue arose from “separable facts”), we reject its general applicability outside of that context.
Thus the application of an estoppel in eases such as this one will require no alteration of this Court’s practice of waiting for conflicts to develop before granting the Government’s petitions for certiorari, nor in the Solicitor General’s policy of circumspection in determining when to pursue appeals or file certiorari petitions. See United States v. Mendoza, ante, p. 154.
The Government argues, however, that in deciding whether to appeal an adverse decision, the Solicitor General has no way of knowing whether future litigation will arise with the same or a different party. The Government thus argues that the mere possibility of being bound in the future will influence the Solicitor General to appeal or seek certiorari from adverse decisions when such action would otherwise be unwarranted. The Government lists as an example Stauffer I, from which the Government did not seek certiorari because there was no circuit conflict at the time of the Tenth Circuit’s decision. Yet, taking the issue here as an example, the Government itself asserts that “thousands of businesses are affected each year by the question of contractor participation in Section 114 inspections.” Brief for United States 28. It is thus unrealistic to assume that the Government would be driven to pursue an unwarranted appeal here because of fear of being unable to relitigate the § 114 issue in the future with a different one of those thousands of affected parties.