BRENNAN, J., dissenting 437 U.S. discretionary that Brillhart found the District Court's deference to state-court proceedings permissible. This is clear from the lower court cases approvingly cited by Brillhart-American Automobile Insurance Co. v. Freundt, 103 F. 2d 613 (CA7 1939); Maryland Casualty Co. v. Consumers Finance Service, 101 F. 2d 514 (CA3 1938); and Aetna Casualty Co. v. Quarles, 92 F. 2d 321 (CA4 1937)—all of which emphasized that a district court's discretion to dismiss a federal declaratory judgment suit in favor of a pending state suit is a product of the permissive nature of declaratory judgment jurisdiction. Obviously neither the logic nor the holding of Brillhart is pertinent where, as here, federal jurisdiction is not only nondiscretionary, but exclusive. 2 The unpersuasive grope for supporting precedent in which the opinion of my Brother REHNQUIST engages is especially lamentable in light of our decision only two Terms ago in Colorado River Water Conservation Dist. v. United States. In Colorado River we addressed the precise issue presented here: the circumstances in which it is appropriate for a federal district court to stay a proceeding before it in deference to a parallel state-court proceeding in situations falling within none of the traditional categories for federal abstention. We explained that, in contrast to situations in which jurisdiction is concurrent in two or more federal courts, 2 These decisions recognized, however, that even where a federal suit seeks only declaratory relief, a district court does not have unbridled authority to dismiss the action in deference to a concurrent state suit. For example, the court in Maryland Casualty Co. v. Consumers Finance Service, 101 F. 2d, at 515, observed: "The granting of the remedy of a declaratory judgment is . . . discretionary with the court and it may be refused if it will not finally settle the rights of the parties or if it is being sought merely to determine issues involved in cases already pending. Aetna Casualty & Surety Co. v. Quarles, 4 Cir., 92 F. 2d 321. It may not be refused, however, merely on the ground that another remedy is available... or because of the pendency of another suit, if the controversy between the parties will not necessarily be determined in that suit." 655 BRENNAN, J., dissenting where the action paralleling a federal suit is in a state court, the federal court's power to dismiss the suit before it in deference to the parallel proceeding is limited by the "virtually unflagging obligation of the federal courts to exercise the jurisdiction given them." 424 U. S., at 817. Because a federal district court's power is so limited, the circumstances that justify federal-court inaction in deference to a state proceeding must be "exceptional." Id., at 818. Just how "exceptional" such circumstances must be was made clear by our admonition that "the circumstances permitting the dismissal of a federal suit due to the presence of a concurrent state proceeding for reasons of wise judicial administration are considerably more limited than the circumstances appropriate for abstention." Ibid. Since we had previously noted that "[a]bdication of the obligation to decide cases can be justified under [the abstention] doctrine only in the exceptional circumstances where the order to the parties to repair to the State court would clearly serve an important countervailing interest,'" id., at 813, quoting County of Allegheny v. Frank Mashuda Co., 360 U. S. 185, 188-189 (1959), the circumstances warranting dismissal "for reasons of wise judicial administration" must be rare indeed. Such rare circumstances were present in Colorado River. There, the decisive factor in favor of staying the concurrent federal proceedings was "[t]he clear federal policy," evinced by the McCarran Amendment, of "avoid [ing the] piecemeal adjudication of water rights in a river system. . . a policy that recognizes the availability of comprehensive state systems for adjudication of water rights as the means for achieving [this] goa[1]." 424 U. S., at 819. No comparable federal policy favoring unitary state adjudication exists here. In fact, as evinced by the exclusive jurisdiction of the federal courts to determine 1934 Act claims, the relevant federal policy here is the precise opposite of that found to require deference to the concurrent state proceeding in Colorado River. BRENNAN, J., dissenting 437 U.S. Ignoring wholesale the analytical framework set forth in Colorado River, whose vitality is not questioned, the opinion of my Brother REHNQUIST seemingly focuses on one of the four secondary factors found to support the federal dismissal in that case the fact that the state proceedings were initiated before the federal suit-and finds that factor sufficient to insulate the District Court's actions here from mandamus review. Even putting aside the opinion's case-reading errors— its flouting of McClellan, its misreliance on Brillhart, and its misapplication of Colorado River—and analyzing this case on the opinion's own erroneous terms, the conclusion is still compelled that the District Court had no authority to stay Calvert's 1934 Act claims. Quite conveniently, the opinion of my Brother REHNQUIST avoids any discussion of the possible res judicata or collateral-estoppel effects the state court's determination of Calvert's 1934 Act defense would have on Calvert's 1934 Act claims for affirmative relief in federal court." To be sure, the preclusive effect of a state-court determination of a claim within the exclusive jurisdiction of the federal courts is an unresolved and difficult issue. See generally Note, Res Judicata: Exclusive Federal Jurisdiction and the Effect of Prior State-Court Determinations, 53 Va. L. Rev. 1360 (1967). For myself, I confess to serious doubt that it is ever appropriate to accord res judicata effect to a state-court determination of a claim over which the federal courts have exclusive jurisdiction; for surely state-court determinations should not disable federal courts from ruling de novo on purely legal questions surrounding such federal claims. See Cotler v. Inter 3 Because the Court of Appeals held that "the district court should not have deferred to the state court on grounds of federalism in light of Colorado River," it found it unnecessary to "reach the difficult issue of whether the conclusion of the state proceedings would have a collateral estoppel effect on the Rule 10b-5 claim for damages over which the court had retained jurisdiction but declined to resolve." 560 F. 2d 792, 797. 655 BRENNAN, J., dissenting County Orthopaedic Assn., 526 F. 2d 537 (CA3 1975); McGough v. First Arlington National Bank, 519 F. 2d 552 (CA7 1975); Clark v. Watchie, 513 F. 2d 994 (CA9 1975). As recognized by Judge Learned Hand in Lyons v. Westinghouse Electric Co., 222 F. 2d 184, 189 (CA2 1955), "the grant to the district courts of exclusive jurisdiction over the action. . should be taken to imply an immunity of their decisions from any prejudgment elsewhere." I recognize that it may make sense, for reasons of fairness and judicial economy, to give collateral-estoppel effect to specific findings of historical facts by a state court's adjudicating an exclusively federal claim raised as a defense, see Granader v. Public Bank, 417 F. 2d 75 (CA6 1969), but there are reasons why even such a limited preclusive effect should not be given state-court determinations. It is at least arguable that, in creating and defining a particular federal claim, Congress assumed that the claim would be litigated only in the context of federal-court procedure a fair assumption when the claim is within exclusive federal jurisdiction. For example, Congress may have thought the liberal federal discovery procedures crucial to the proper determination of the factual disputes underlying the federal claim. All this is not to say that I disagree with the refusal of the opinion of my Brother REHNQUIST to decide what preclusive effects the state court's determination of Calvert's Rule 10b-5 defense would have in Calvert's federal action, so much as it is to expose the opinion's error in failing even to consider the res judicata/collateral estoppel problem in evaluating the District Court's obligation to adjudicate Calvert's Rule 10b-5 claim. In my view, regardless of whether the state-court judgment would be given res judicata or collateral-estoppel effect, it was incumbent upon the District Court at least in the absence of other overriding reasons-expeditiously to adjudicate at least Calvert's 1934 Act claims. If res judicata effect is accorded the prior state-court judgment, the exclusive jurisdic BRENNAN, J., dissenting 437 U.S. tion given the federal courts over 1934 Act claims would be effectively thwarted, and the policy of uniform and effective federal administration and interpretation of the 1934 Act frustrated. A stay having so undesirable a consequence could possibly be justified only by compelling circumstances absent here. On the other hand, if the state-court adjudication is not given res judicata or collateral-estoppel effect, the 1934 Act claims will have to be adjudicated in federal court in any event, and there would be no reason for staying the federal action since nothing that transpires in the state proceedings would affect the adjudication of the federal claims. Thus, regardless of the proper disposition of the res judicata/collateral estoppel question, it is clear that a district court should not stay claims over which the federal courts have exclusive jurisdiction. See Cotler v. Inter-County Orthopaedic Assn., supra; Lecor, Inc. v. United States District Court, 502 F. 2d 104 (CA9 1974). II Whether evaluated under the "clear abuse of discretion" standard set forth in La Buy v. Howes Leather Co., 352 U. S. 249, 257 (1957), or under the prong of Will v. United States, 389 U. S. 90, 95 (1967), and Roche v. Evaporated Milk Assn., 319 U. S. 21, 26 (1943), that permits the use of mandamus "to compel [an inferior court] to exercise its authority when it is its duty to do so," the issuance of the writ of mandamus by the Court of Appeals was proper; there is simply a complete dearth of "exceptional" circumstances countervailing the District Court's "unflagging obligation" to exercise its exclusive jurisdiction. The opinion of my Brother REHNQUIST asserts, however, that the District Court "has not purported to stay consideration of Calvert's claim for damages under the Securities Exchange Act of 1934," but rather has simply "not yet ruled upon this claim." Ante, at 666. While technically accurate, this characterization of the status of the proceedings below utterly ignores two important facts that shed more than |