(Slip Opinion) OCTOBER TERM, 2010 1 

Syllabus 

NOTE: Where it is feasible, a syllabus (headnote) will be released, as is

being done in connection with this case, at the time the opinion is issued.

The syllabus constitutes no part of the opinion of the Court but has been

prepared by the Reporter of Decisions for the convenience of the reader. 

See United States v. Detroit Timber & Lumber Co., 200 U. S. 321, 337. 

SUPREME COURT OF THE UNITED STATES 

Syllabus 

SMITH ET AL. v. BAYER CORP. 

CERTIORARI TO THE UNITED STATES COURT OF APPEALS FOR 

THE EIGHTH CIRCUIT 

No. 09–1205. Argued January 18, 2011—Decided June 16, 2011 

Respondent (Bayer) moved in Federal District Court for an injunction 

ordering a West Virginia state court not to consider a motion for class 

certification filed by petitioners (Smith), who were plaintiffs in the

state-court action. Bayer thought such an injunction warranted because, in a separate case, Bayer had persuaded the same Federal 

District Court to deny a similar class-certification motion that had 

been filed against Bayer by a different plaintiff, George McCollins. 

The District Court had denied McCollins’ certification motion under 

Fed. Rule Civ. Proc. 23. 

The court granted Bayer’s requested injunction against the state

court proceedings, holding that its denial of certification in McCollins’

case precluded litigation of the certification issue in Smith’s case. 

The Court of Appeals for the Eighth Circuit affirmed. It first noted 

that the Anti-Injunction Act (Act) generally prohibits federal courts 

from enjoining state court proceedings. But it found that the Act’s relitigation exception authorized this injunction because ordinary rules

of issue preclusion barred Smith from seeking certification of his proposed class. In so doing, the court concluded that Smith was invoking a State Rule, W. Va. Rule Civ. Proc. 23, that was sufficiently 

similar to the Federal Rule McCollins had invoked, such that the certification issues presented in the two cases were the same. The court 

further held that Smith, as an unnamed member of McCollins’ putative class action, could be bound by the judgment in McCollins’ case. 

Held: In enjoining the state court from considering Smith’s class certification request, the federal court exceeded its authority under the “relitigation exception” to the Act. Pp. 5–18.

(a) Under that Act, a federal court “may not grant an injunction to 

stay proceedings in a State court except” in rare cases, when neces-
2 SMITH v. BAYER CORP. 

Syllabus 

sary to “protect or effectuate [the federal court’s] judgments.” 28 

U. S. C. §2283. The Act’s “specifically defined exceptions,” Atlantic 

Coast Line R. Co. v. Locomotive Engineers, 398 U. S. 281, 286, “are 

narrow and are ‘not [to] be enlarged by loose statutory construction,’ ” 

Chick Kam Choo v. Exxon Corp., 486 U. S. 140, 146. Indeed, “[a]ny

doubts as to the propriety of a federal injunction against state court 

proceedings should be resolved in favor of permitting the state courts

to proceed.” Atlantic Coast Line R. Co., 398 U. S., at 297. The exception at issue in this case, known as the “relitigation exception,” authorizes an injunction to prevent state litigation of a claim or issue

“that previously was presented to and decided by the federal court.” 

Chick Kam Choo, 486 U. S., at 147. This exception is designed to implement “well-recognized concepts” of claim and issue preclusion. 

Ibid. Because deciding whether and how prior litigation has preclusive effect is usually the bailiwick of the second court—here, the West 

Virginia court—every benefit of the doubt goes toward the state 

court, see Atlantic Coast Line, 398 U. S., at 287, 297; an injunction

can issue only if preclusion is clear beyond peradventure. For the 

federal court’s class-action determination to preclude the state court’s 

adjudication of Smith’s motion, at least two conditions must be met.

First, the issue the federal court decided must be the same as the one 

presented in the state tribunal. And second, Smith must have been a 

party to the federal suit or must fall within one of a few discrete exceptions to the general rule against binding nonparties. Pp. 5–7.

(b) The issue the federal court decided was not the same as the one 

presented in the state tribunal. This case is little more than a rerun 

of Chick Kam Choo. There, a federal court dismissed a suit involving 

Singapore law on forum non conveniens grounds and then enjoined

the plaintiff from pursuing the “same” claim in Texas state court. 

However, because the legal standards for forum non conveniens differed in the two courts, the issues before those courts differed, making an injunction unwarranted. Here, Smith’s proposed class mirrored McCollins’, and the two suits’ substantive claims broadly

overlapped. But the federal court adjudicated McCollins’ certification

motion under Federal Rule 23, whereas the state court was poised to

consider Smith’s proposed class under W. Va. Rule 23. And the State 

Supreme Court has generally stated that it will not necessarily interpret its Rule 23 as coterminous with the Federal Rule. Absent clear 

evidence that the state courts had adopted an approach to State Rule

23 tracking the federal court’s analysis in McCollins’ case, this Court 

could not conclude that they would interpret their Rule the same way

and, thus, could not tell whether the certification issues in the two 

courts were the same. That uncertainty would preclude an injunction. And indeed, the case against an injunction here is even 
Cite as: 564 U. S. ____ (2011) 3 

Syllabus 

stronger, because the State Supreme Court has expressly disapproved the approach to Rule 23(b)(3)’s predominance requirement 

embraced by the Federal District Court. Pp. 8–12.

(c) The District Court’s injunction was independently improper because Smith was not a party to the federal suit and was not covered 

by any exception to the rule against nonparty preclusion. Generally, 

a party “is ‘[o]ne by or against whom a lawsuit is brought,’ ” United 

States ex rel. Eisenstein v. City of New York, 556 U. S. ___, ___, or 

who “become[s] a party by intervention, substitution, or third-party

practice,” Karcher v. May, 484 U. S. 72, 77. The definition of “party”

cannot be stretched so far as to cover a person like Smith, whom

McCollins was denied leave to represent. The only exception to the

rule against nonparty preclusion potentially relevant here is the exception that binds non-named members of “properly conducted class 

actions” to judgments entered in such proceedings. Taylor v. Sturgell, 553 U. S. 880, 894. But McCollins’ suit was not a proper class 

action. Indeed, the very ruling that Bayer argues should have preclusive effect is the District Court’s decision not to certify a class.

Absent certification of a class under Federal Rule 23, the precondition for binding Smith was not met. Neither a proposed, nor a rejected, class action may bind nonparties. See id., at 901. Bayer

claims that this Court’s approach to class actions would permit class

counsel to try repeatedly to certify the same class simply by changing

plaintiffs. But principles of stare decisis and comity among courts 

generally suffice to mitigate the sometimes substantial costs of similar litigation brought by different plaintiffs. The right approach does 

not lie in binding nonparties to a judgment. And to the extent class 

actions raise special relitigation problems, the federal Class Action

Fairness Act of 2005 provides a remedy that does not involve departing from the usual preclusion rules. Pp. 12–18. 

593 F. 3d 716, reversed. 

KAGAN, J., delivered the opinion of the Court, in which ROBERTS, 

C. J., and SCALIA, KENNEDY, GINSBURG, BREYER, ALITO, and SOTOMAYOR, 

JJ., joined, and in which THOMAS, J., joined as to Parts I and II–A. 
_________________ 

_________________ 

Cite as: 564 U. S. ____ (2011) 1 

Opinion of the Court 

NOTICE: This opinion is subject to formal revision before publication in the

preliminary print of the United States Reports. Readers are requested to

notify the Reporter of Decisions, Supreme Court of the United States, Washington, D. C. 20543, of any typographical or other formal errors, in order

that corrections may be made before the preliminary print goes to press. 

SUPREME COURT OF THE UNITED STATES 

No. 09–1205 

KEITH SMITH, ET AL., PETITIONERS v. BAYER 

CORPORATION

ON WRIT OF CERTIORARI TO THE UNITED STATES COURT OF 

APPEALS FOR THE EIGHTH CIRCUIT

[June 16, 2011] 

JUSTICE KAGAN delivered the opinion of the Court.* 

In this case, a Federal District Court enjoined a state

court from considering a plaintiff’s request to approve a

class action. The District Court did so because it had 

earlier denied a motion to certify a class in a related case,

brought by a different plaintiff against the same defendant alleging similar claims. The federal court thought its

injunction appropriate to prevent relitigation of the issue

it had decided. 

We hold to the contrary. In issuing this order to a state 

court, the federal court exceeded its authority under the

“relitigation exception” to the Anti-Injunction Act. That 

statutory provision permits a federal court to enjoin a

state proceeding only in rare cases, when necessary to “protect or effectuate [the federal court’s] judgments.” 28 

U. S. C. §2283. Here, that standard was not met for two 

reasons. First, the issue presented in the state court was

not identical to the one decided in the federal tribunal. 

And second, the plaintiff in the state court did not have

the requisite connection to the federal suit to be bound by 

—————— 

* JUSTICE THOMAS joins Parts I and II–A of this opinion. 
2 SMITH v. BAYER CORP. 

Opinion of the Court 

the District Court’s judgment. 

I 

Because the question before us involves the effect of a 

former adjudication on this case, we begin our statement 

of the facts not with this lawsuit, but with another. In 

August 2001, George McCollins sued respondent Bayer 

Corporation in the Circuit Court of Cabell County, West 

Virginia, asserting various state-law claims arising from 

Bayer’s sale of an allegedly hazardous prescription drug 

called Baycol (which Bayer withdrew from the market that

same month). McCollins contended that Bayer had violated West Virginia’s consumer-protection statute and the 

company’s express and implied warranties by selling him 

a defective product. And pursuant to West Virginia Rule 

of Civil Procedure 23 (2011), McCollins asked the state

court to certify a class of West Virginia residents who had

also purchased Baycol, so that the case could proceed as a

class action. 

Approximately one month later, the suit now before us 

began in a different part of West Virginia. Petitioners 

Keith Smith and Shirley Sperlazza (Smith for short) filed

state-law claims against Bayer, similar to those raised in 

McCollins’ suit, in the Circuit Court of Brooke County, 

West Virginia. And like McCollins, Smith asked the court 

to certify under West Virginia’s Rule 23 a class of Baycol

purchasers residing in the State. Neither Smith nor 

McCollins knew about the other’s suit. 

In January 2002, Bayer removed McCollins’ case to the 

United States District Court for the Southern District of 

West Virginia on the basis of diversity jurisdiction. See 28 

U. S. C. §§1332, 1441. The case was then transferred to 

the District of Minnesota pursuant to a preexisting order

of the Judicial Panel on Multi-District Litigation, which 

had consolidated all federal suits involving Baycol (numbering in the tens of thousands) before a single District 
Cite as: 564 U. S. ____ (2011) 3 

Opinion of the Court 

Court Judge. See §1407. Bayer, however, could not 

remove Smith’s case to federal court because Smith 

had sued several West Virginia defendants in addition to 

Bayer, and so the suit lacked complete diversity. See 

§1441(b).1 Smith’s suit thus remained in the state courthouse in Brooke County. 

Over the next six years, the two cases proceeded along 

their separate pretrial paths at roughly the same pace. By

2008, both courts were preparing to turn to their respective plaintiffs’ motions for class certification. The Federal 

District Court was the first to reach a decision. 

Applying Federal Rule of Civil Procedure 23,2 the District Court declined to certify McCollins’ proposed class

of West Virginia Baycol purchasers. The District Court’s 

reasoning proceeded in two steps. The court first ruled 

that, under West Virginia law, each plaintiff would have

to prove “actual injury” from his use of Baycol to recover.

App. to Pet. for Cert. 44a. The court then held that because the necessary showing of harm would vary from

plaintiff to plaintiff, “individual issues of fact predominate[d]” over issues common to all members of the proposed class, and so the case was not suitable for class 

treatment. Id., at 45a. In the same order, the District 

Court also dismissed McCollins’ claims on the merits in 

light of his failure to demonstrate physical injury from his

use of Baycol. McCollins chose not to appeal. 

Although McCollins’ suit was now concluded, Bayer 

asked the District Court for another order based upon it, 

—————— 

1The Class Action Fairness Act of 2005, 119 Stat. 4, which postdates

and therefore does not govern this lawsuit, now enables a defendant to 

remove to federal court certain class actions involving nondiverse 

parties. See 28 U. S. C. §§1332(d), 1453(b); see also infra, at 17. 2Although McCollins had originally sought certification under West 

Virginia Rule of Civil Procedure 23 (2011), federal procedural rules 

govern a case that has been removed to federal court. See Shady Grove 

Orthopedic Associates, P. A. v. Allstate Ins. Co., 559 U. S. ___ (2010). 
4 SMITH v. BAYER CORP. 

Opinion of the Court 

this one affecting Smith’s case in West Virginia. In a 

motion—receipt of which first apprised Smith of McCollins’ suit—Bayer explained that the proposed class in 

Smith’s case was identical to the one the federal court had 

just rejected. Bayer therefore requested that the federal

court enjoin the West Virginia state court from hearing

Smith’s motion to certify a class. According to Bayer, that

order was appropriate to protect the District Court’s

judgment in McCollins’ suit denying class certification. 

The District Court agreed and granted the injunction.

The Court of Appeals for the Eighth Circuit affirmed. 

In re Baycol Prods. Litigation, 593 F. 3d 716 (2010). The 

court noted that the Anti-Injunction Act generally prohibits federal courts from enjoining state court proceedings.

But the court held that the Act’s relitigation exception

authorized the injunction here because ordinary rules of 

issue preclusion barred Smith from seeking certification of 

his proposed class. According to the court, Smith was

invoking a similar class action rule as McCollins had used 

to seek certification “of the same class” in a suit alleging 

“the same legal theories,” id., at 724; the issue in the state 

court therefore was “sufficiently identical” to the one the 

federal court had decided to warrant preclusion, ibid. In 

addition, the court held, the parties in the two proceedings

were sufficiently alike: Because Smith was an unnamed

member of the class McCollins had proposed, and because 

their “interests were aligned,” Smith was appropriately

bound by the federal court’s judgment. Ibid. 

We granted certiorari, 561 U. S. __ (2010), because the 

order issued here implicates two circuit splits arising from 

application of the Anti-Injunction Act’s relitigation exception. The first involves the requirement of preclusion law

that a subsequent suit raise the “same issue” as a previous 

case.3 The second concerns the scope of the rule that a 

—————— 

3Compare In re Baycol Prods. Litigation, 593 F. 3d 716, 723 (CA8 
Cite as: 564 U. S. ____ (2011) 5 

Opinion of the Court 

court’s judgment cannot bind nonparties.4 We think the 

District Court erred on both grounds when it granted the 

injunction, and we now reverse. 

II 

The Anti-Injunction Act, first enacted in 1793, provides

that 

“A court of the United States may not grant an injunction to stay proceedings in a State court except as

expressly authorized by Act of Congress, or where 

necessary in aid of its jurisdiction, or to protect or

effectuate its judgments.” 28 U. S. C. §2283. 

The statute, we have recognized, “is a necessary concomitant of the Framers’ decision to authorize, and Congress’ 

decision to implement, a dual system of federal and state

courts.” Chick Kam Choo v. Exxon Corp., 486 U. S. 140, 

146 (1988). And the Act’s core message is one of respect 

for state courts. The Act broadly commands that those 

tribunals “shall remain free from interference by federal 

courts.” Atlantic Coast Line R. Co. v. Locomotive Engi-

—————— 

2010) (case below) (holding that two cases involve the same issue when 

“[t]he state and federal [class] certification rules . . . are not significantly different”), with J. R. Clearwater Inc. v. Ashland Chemical Co., 

93 F. 3d 176, 180 (CA5 1996) (holding that two cases implicate different

issues even when “[the state rule] is modeled on . . . the Federal Rules”

because a “[state] court might well exercise [its] discretion in a different

manner”). 

4Compare 593 F. 3d, at 724 (“[T]he denial of class certification is 

binding on unnamed [putative] class members” because they are “in 

privity to [the parties] in the prior action”) and In re Bridgestone/Firestone, Inc., Tires Prods. Liability Litigation, 333 F. 3d 763, 

768–769 (CA7 2003) (same), with In re Ford Motor Co., 471 F. 3d 1233, 

1245 (CA11 2006) (holding that “[t]he denial of class certification” 

prevents a court from “binding” anyone other than “the parties appearing before it”) and In re General Motors Corp. Pick-Up Truck Fuel Tank 

Prods. Liability Litigation, 134 F. 3d 133, 141 (CA3 1998) (holding that

putative “class members are not parties” and so cannot be bound by a

court’s ruling when “there is no class pending”). 
6 SMITH v. BAYER CORP. 

Opinion of the Court 

neers, 398 U. S. 281, 282 (1970). That edict is subject to

only “three specifically defined exceptions.” Id., at 286. 

And those exceptions, though designed for important 

purposes, “are narrow and are ‘not [to] be enlarged by

loose statutory construction.’” Chick Kam Choo, 486 U. S., 

at 146 (quoting Atlantic Coast Line, 398 U. S., at 287; 

alteration in original). Indeed, “[a]ny doubts as to the

propriety of a federal injunction against state court proceedings should be resolved in favor of permitting the 

state courts to proceed.” Id., at 297. 

This case involves the last of the Act’s three exceptions, 

known as the relitigation exception. That exception is designed to implement “well-recognized concepts” of claim

and issue preclusion. Chick Kam Choo, 486 U. S., at 147. 

The provision authorizes an injunction to prevent state

litigation of a claim or issue “that previously was presented to and decided by the federal court.” Ibid. But in 

applying this exception, we have taken special care to 

keep it “strict and narrow.” Id., at 148. After all, a court 

does not usually “get to dictate to other courts the preclusion consequences of its own judgment.” 18 C. Wright,

A. Miller, & E. Cooper, Federal Practice and Procedure 

§4405, p. 82 (2d ed. 2002) (hereinafter Wright & Miller). 

Deciding whether and how prior litigation has preclusive 

effect is usually the bailiwick of the second court (here, the 

one in West Virginia). So issuing an injunction under the

relitigation exception is resorting to heavy artillery.5 For 

that reason, every benefit of the doubt goes toward the 

state court, see Atlantic Coast Line, 398 U. S., at 287, 297; 

an injunction can issue only if preclusion is clear beyond 

—————— 

5That is especially so because an injunction is not the only way to 

correct a state trial court’s erroneous refusal to give preclusive effect to 

a federal judgment. As we have noted before, “the state appellate

courts and ultimately this Court” can review and reverse such a ruling.

See Atlantic Coast Line R. Co. v. Locomotive Engineers, 398 U. S. 281, 

287 (1970). 
Cite as: 564 U. S. ____ (2011) 7 

Opinion of the Court 

peradventure. 

The question here is whether the federal court’s rejection of McCollins’ proposed class precluded a later adjudication in state court of Smith’s certification motion. For 

the federal court’s determination of the class issue to have 

this preclusive effect, at least two conditions must be met.6 

First, the issue the federal court decided must be the same 

as the one presented in the state tribunal. See 18 Wright

& Miller §4417, at 412. And second, Smith must have 

been a party to the federal suit, or else must fall within

one of a few discrete exceptions to the general rule against 

binding nonparties. See 18A id., §4449, at 330. In fact, 

as we will explain, the issues before the two courts were 

not the same, and Smith was neither a party nor the exceptional kind of nonparty who can be bound. So the 

courts below erred in finding the certification issue precluded, and erred all the more in thinking an injunction

appropriate.7 

—————— 

6We have held that federal common law governs the preclusive effect 

of a decision of a federal court sitting in diversity. See Semtek Int’l Inc. 

v. Lockheed Martin Corp., 531 U. S. 497, 508 (2001). Smith assumes 

that federal common law should here incorporate West Virginia’s 

preclusion law, see Brief for Petitioners 15–16, whereas Bayer favors 

looking only to federal rules of preclusion because of the federal interests at stake in this case, see Brief for Respondent 18. We do not think 

the question matters here. Neither party identifies any way in which 

federal and state principles of preclusion law differ in any relevant 

respect. Nor have we found any such divergence. Compare, e.g., 

Montana v. United States, 440 U. S. 147, 153–154 (1979) (describing

elements of issue preclusion), with State v. Miller, 194 W. Va. 3, 9, 459 

S. E. 2d 114, 120 (1995) (same). We therefore need not decide whether, 

in general, federal common law ought to incorporate state law in

situations such as this. 7Because we rest our decision on the Anti-Injunction Act and the 

principles of issue preclusion that inform it, we do not consider Smith’s

argument, based on Phillips Petroleum Co. v. Shutts, 472 U. S. 797 

(1985), that the District Court’s action violated the Due Process Clause. 
8 SMITH v. BAYER CORP. 

Opinion of the Court 

A 

In our most recent case on the relitigation exception, 

Chick Kam Choo v. Exxon, we applied the “same issue” 

requirement of preclusion law to invalidate a federal 

court’s injunction. 486 U. S., at 151. The federal court 

had dismissed a suit involving Singapore law on grounds

of forum non conveniens. After the plaintiff brought the 

same claim in Texas state court, the federal court issued 

an injunction barring the plaintiff from pursuing relief in 

that alternate forum. We held that the District Court had 

gone too far. “[A]n essential prerequisite for applying the 

relitigation exception,” we explained, “is that the . . . issues which the federal injunction insulates from litigation 

in state proceedings actually have been decided by the

federal court.” Id., at 148. That prerequisite, we thought,

was not satisfied because the issue to be adjudicated in 

state court was not the one the federal court had resolved. 

The federal court had considered the permissibility of the

claim under federal forum non conveniens principles. But 

the Texas courts, we thought, “would apply a significantly

different forum non conveniens analysis,” id., at 149; they 

had in prior cases rejected the strictness of the federal

doctrine. Our conclusion followed: “[W]hether the Texas 

state courts are an appropriate forum for [the plaintiff’s] 

Singapore law claims has not yet been litigated.” Ibid. 

Because the legal standards in the two courts differed, the 

issues before the courts differed, and an injunction was

unwarranted. 

The question here closely resembles the one in Chick 

Kam Choo. The class Smith proposed in state court mirrored the class McCollins sought to certify in federal court: 

Both included all Baycol purchasers resident in West 

Virginia. Moreover, the substantive claims in the two 

suits broadly overlapped: Both complaints alleged that 

Bayer had sold a defective product in violation of the

State’s consumer protection law and the company’s war-
Cite as: 564 U. S. ____ (2011) 9 

Opinion of the Court 

ranties. So far, so good for preclusion. But not so fast: 

a critical question—the question of the applicable legal 

standard—remains. The District Court ruled that the 

proposed class did not meet the requirements of Federal 

Rule 23 (because individualized issues would predominate 

over common ones). But the state court was poised to

consider whether the proposed class satisfied West Virginia Rule 23. If those two legal standards differ (as

federal and state forum non conveniens law differed in 

Chick Kam Choo)—then the federal court resolved an 

issue not before the state court. In that event, much like 

in Chick Kam Choo, “whether the [West Virginia] state 

cour[t]” should certify the proposed class action “has not

yet been litigated.” 486 U. S., at 149. 

The Court of Appeals and Smith offer us two competing 

ways of deciding whether the West Virginia and Federal 

Rules differ, but we think the right path lies somewhere in

the middle. The Eighth Circuit relied almost exclusively 

on the near-identity of the two Rules’ texts. See 593 F. 3d, 

at 723. That was the right place to start, but not to end. 

Federal and state courts, after all, can and do apply identically worded procedural provisions in widely varying 

ways. If a State’s procedural provision tracks the language of a Federal Rule, but a state court interprets that

provision in a manner federal courts have not, then the

state court is using a different standard and thus deciding

a different issue. See 18 Wright & Miller §4417, at 454

(stating that preclusion is “inappropriate” when “different 

legal standards . . . masquerad[e] behind similar legal 

labels”). At the other extreme, Smith contends that the 

source of law is all that matters: a different sovereign

must in each and every case “have the opportunity, if it

chooses, to construe its procedural rule differently.” Brief 

for Petitioners 22 (quoting ALI, Principles of the Law,

Aggregate Litigation §2.11, Reporters’ Notes, cmt. b, 

p. 181 (2010)). But if state courts have made crystal clear 
10 SMITH v. BAYER CORP. 

Opinion of the Court 

that they follow the same approach as the federal court

applied, we see no need to ignore that determination; in

that event, the issues in the two cases would indeed be the 

same. So a federal court considering whether the relitigation exception applies should examine whether state law 

parallels its federal counterpart. But as suggested earlier, 

see supra, at 6, the federal court must resolve any uncertainty on that score by leaving the question of preclusion 

to the state courts. 

Under this approach, the West Virginia Supreme Court 

has gone some way toward resolving the matter before us 

by declaring its independence from federal courts’ interpretation of the Federal Rules—and particularly of Rule 

23. In In re W. Va. Rezulin Litigation, 214 W. Va. 52, 585 

S. E. 2d 52 (2003) (In re Rezulin), the West Virginia high 

court considered a plaintiff’s motion to certify a class—

coincidentally enough, in a suit about an allegedly defective pharmaceutical product. The court made a point of

complaining about the parties’ and lower court’s nearexclusive reliance on federal cases about Federal Rule 23 

to decide the certification question. Such cases, the court 

cautioned, “‘may be persuasive, but [they are] not binding 

or controlling.’” Id., at 61, 585 S. E. 2d, at 61. And lest 

anyone mistake the import of this message, the court went

on: The aim of “this rule is to avoid having our legal 

analysis of our Rules ‘amount to nothing more than Pavlovian responses to federal decisional law.’” Ibid. (italics

omitted). Of course, the state courts might still have

adopted an approach to their Rule 23 that tracked the

analysis the federal court used in McCollins’ case. But 

absent clear evidence that the state courts had done so, we 

could not conclude that they would interpret their Rule in

the same way. And if that is so, we could not tell whether 

the certification issues in the state and federal courts were 

the same. That uncertainty would preclude an injunction.

But here the case against an injunction is even stronger, 
Cite as: 564 U. S. ____ (2011) 11 

Opinion of the Court 

because the West Virginia Supreme Court has disapproved the approach to Rule 23(b)(3)’s predominance

requirement that the Federal District Court embraced.

Recall that the federal court held that the presence of 

a single individualized issue—injury from the use of 

Baycol—prevented class certification. See supra, at 3. The 

court did not identify the common issues in the case; nor

did it balance these common issues against the need to

prove individual injury to determine which predominated. 

The court instead applied a strict test barring class treatment when proof of each plaintiff’s injury is necessary.8 

By contrast, the West Virginia Supreme Court in In 

re Rezulin adopted an all-things-considered, balancing 

inquiry in interpreting its Rule 23. Rejecting any “rigid 

test,” the state court opined that the predominance requirement “contemplates a review of many factors.” 214 

W. Va., at 72, 585 S. E. 2d, at 72. Indeed, the court noted, 

a “‘single common issue’” in a case could outweigh “‘numerous . . . individual questions.’” Ibid. That meant, the 

court further explained (quoting what it termed the “leading treatise” on the subject), that even objections to certification “‘based on . . . causation, or reliance’”—which typically involve showings of individual injury—“‘will not bar 

predominance satisfaction.’” Ibid. (quoting 2 A. Conte & 

H. Newberg, Newberg on Class Actions §4.26, p. 241 (4th

ed. 2002)). So point for point, the analysis set out in In re 

Rezulin diverged from the District Court’s interpretation

of Federal Rule 23. A state court using the In re Rezulin 

standard would decide a different question than the one 

—————— 

8The District Court’s approach to the predominance inquiry is consistent with the approach employed by the Eighth Circuit. See In re St. 

Jude Medical, Inc., 522 F. 3d 836, 837–840 (2008) (holding that most

commercial misrepresentation cases are “unsuitable for class treatment” because individual issues of reliance necessarily predominate). 

We express no opinion as to the correctness of this approach. 
12 SMITH v. BAYER CORP. 

Opinion of the Court 

the federal court had earlier resolved.9 

This case, indeed, is little more than a rerun of Chick 

Kam Choo. A federal court and a state court apply different law. That means they decide distinct questions. The 

federal court’s resolution of one issue does not preclude the 

state court’s determination of another. It then goes without saying that the federal court may not issue an injunction. The Anti-Injunction Act’s re-litigation exception does

not extend nearly so far. 

B 

The injunction issued here runs into another basic 

premise of preclusion law: A court’s judgment binds only 

the parties to a suit, subject to a handful of discrete and 

limited exceptions. See, e.g., 18A Wright & Miller §4449, 

at 330. The importance of this rule and the narrowness of 

its exceptions go hand in hand. We have repeatedly “emphasize[d] the fundamental nature of the general rule” 

that only parties can be bound by prior judgments; accordingly, we have taken a “constrained approach to nonparty 

preclusion.” Taylor v. Sturgell, 553 U. S. 880, 898 (2008).

Against this backdrop, Bayer defends the decision below

by arguing that Smith—an unnamed member of a pro-

—————— 

9Bayer argues that In re Rezulin does not preclude an injunction in 

this case because the West Virginia court there decided that common

issues predominated over individual issues of damages, not over individual issues of liability (as exist here). See Brief for Respondent 25–

26. We think Bayer is right about this distinction, but wrong about its 

consequence. Our point is not that In re Rezulin dictates the answer 

to the class certification question here; the two cases are indeed too

dissimilar for that to be true. The point instead is that In re Rezulin 

articulated a general approach to the predominance requirement that 

differs markedly from the one the federal court used. Minor variations 

in the application of what is in essence the same legal standard do not

defeat preclusion; but where, as here, the State’s courts “would apply a

significantly different . . . analysis,” Chick Kam Choo v. Exxon Corp., 

486 U. S. 140, 149 (1988), the federal and state courts decide different

issues. 
Cite as: 564 U. S. ____ (2011) 13 

Opinion of the Court 

posed but uncertified class—qualifies as a party to the 

McCollins litigation. See Brief for Respondent 32–34.

Alternatively, Bayer claims that the District Court’s 

judgment binds Smith under the recognized exception to 

the rule against nonparty preclusion for members of class 

actions. See id., at 34–39. We think neither contention 

has merit. 

Bayer’s first claim ill-comports with any proper understanding of what a “party” is. In general, “[a] ‘party’ to 

litigation is ‘[o]ne by or against whom a lawsuit is

brought,’” United States ex rel. Eisenstein v. City of New 

York, 556 U. S. ___, ___ (2009) (slip op., at 4), or one who 

“become[s] a party by intervention, substitution, or thirdparty practice,” Karcher v. May, 484 U. S. 72, 77 (1987).

And we have further held that an unnamed member of a 

certified class may be “considered a ‘party’ for the [particular] purpos[e] of appealing” an adverse judgment. Devlin 

v. Scardelletti, 536 U. S. 1, 7 (2002). But as the dissent in 

Devlin noted, no one in that case was “willing to advance

the novel and surely erroneous argument that a nonnamed class member is a party to the class-action litigation before the class is certified.” Id., at 16, n. 1 (opinion of 

SCALIA, J.). Still less does that argument make sense once 

certification is denied. The definition of the term “party”

can on no account be stretched so far as to cover a person

like Smith, whom the plaintiff in a lawsuit was denied

leave to represent.10 If the judgment in the McCollins 

—————— 

10 In support of its claim that Smith counts as a party, Bayer cites two 

cases in which we held that a putative member of an uncertified class 

may wait until after the court rules on the certification motion to file an

individual claim or move to intervene in the suit. See Brief for Respondent 32–33 (citing United Airlines, Inc. v. McDonald, 432 U. S. 385 

(1977); American Pipe & Constr. Co. v. Utah, 414 U. S. 538 (1974)). But 

these cases, which were specifically grounded in policies of judicial 

administration, demonstrate only that a person not a party to a class

suit may receive certain benefits (such as the tolling of a limitations

period) related to that proceeding. See id., at 553; McDonald, 432 
14 SMITH v. BAYER CORP. 

Opinion of the Court 

litigation can indeed bind Smith, it must do so under 

principles of nonparty preclusion.

As Bayer notes, see Brief for Respondent 37, one such 

principle allows unnamed members of a class action to be 

bound, even though they are not parties to the suit. See 

Cooper v. Federal Reserve Bank of Richmond, 467 U. S. 

867, 874 (1984) (“[U]nder elementary principles of prior 

adjudication a judgment in a properly entertained class 

action is binding on class members in any subsequent

litigation”); see also Taylor, 553 U. S., at 894 (stating that

nonparties can be bound in “properly conducted class 

actions”). But here Bayer faces a conundrum. If we know 

one thing about the McCollins suit, we know that it was 

not a class action. Indeed, the very ruling that Bayer

argues ought to be given preclusive effect is the District

Court’s decision that a class could not properly be certified. So Bayer wants to bind Smith as a member of a class

action (because it is only as such that a nonparty in

Smith’s situation can be bound) to a determination that 

there could not be a class action. And if the logic of that

position is not immediately transparent, here is Bayer’s

attempt to clarify: “[U]ntil the moment when class certification was denied, the McCollins case was a properly

conducted class action.” Brief for Respondent 37. That is 

true, according to Bayer, because McCollins’ interests

were aligned with the members of the class he proposed 

and he “act[ed] in a representative capacity when he

sought class certification.” Id., at 36. 

But wishing does not make it so. McCollins sought class

certification, but he failed to obtain that result. Because 

the District Court found that individual issues predomi-

—————— 

U. S., at 394, n. 15. That result is consistent with a commonplace of 

preclusion law—that nonparties sometimes may benefit from, even 

though they cannot be bound by, former litigation. See Parklane Hosiery Co. v. Shore, 439 U. S. 322, 326–333 (1979); Blonder-Tongue 

Laboratories, Inc. v. University of Ill. Foundation, 402 U. S. 313 (1971). 
Cite as: 564 U. S. ____ (2011) 15 

Opinion of the Court 

nated, it held that the action did not satisfy Federal Rule

23’s requirements for class proceedings. In these circumstances, we cannot say that a properly conducted class

action existed at any time in the litigation. Federal Rule 

23 determines what is and is not a class action in federal 

court, where McCollins brought his suit. So in the absence 

of a certification under that Rule, the precondition for 

binding Smith was not met. Neither a proposed class 

action nor a rejected class action may bind nonparties.

What does have this effect is a class action approved under

Rule 23. But McCollins’ lawsuit was never that. 

We made essentially these same points in Taylor v. 

Sturgell just a few Terms ago. The question there concerned the propriety of binding nonparties under a theory 

of “virtual representation” based on “identity of interests

and some kind of relationship between parties and nonparties.” 553 U. S., at 901. We rejected the theory

unanimously, explaining that it “would ‘recogniz[e], in 

effect, a common-law kind of class action.’” Ibid. Such a 

device, we objected, would authorize preclusion “shorn of

[Rule 23’s] procedural protections.” Ibid. Or as otherwise 

stated in the opinion: We could not allow “circumvent[ion]” 

of Rule 23’s protections through a “virtual representation 

doctrine that allowed courts to ‘create de facto class actions at will.’” Ibid. We could hardly have been more

clear that a “properly conducted class action,” with binding effect on nonparties, can come about in federal courts 

in just one way—through the procedure set out in Rule 23.

Bayer attempts to distinguish Taylor by noting that the

party in the prior litigation there did not propose a class 

action. But we do not see why that difference matters. 

Yes, McCollins wished to represent a class, and made a

motion to that effect. But it did not come to pass. To 

allow McCollins’ suit to bind nonparties would be to adopt 
16 SMITH v. BAYER CORP. 

Opinion of the Court 

the very theory Taylor rejected.11 

Bayer’s strongest argument comes not from established 

principles of preclusion, but instead from policy concerns 

relating to use of the class action device. Bayer warns

that under our approach class counsel can repeatedly try 

to certify the same class “by the simple expedient of

changing the named plaintiff in the caption of the complaint.” Brief for Respondent 47–48. And in this world of 

“serial relitigation of class certification,” Bayer contends, 

defendants “would be forced in effect to buy litigation

peace by settling.” Id., at 2, 12; see also In re Bridgestone/Firestone, Inc., Tires Prods. Liability Litigation,

333 F. 3d 763, 767 (CA7 2003) (objecting to an “an asymmetric system in which class counsel can win but never

lose” because of their ability to relitigate the issue of 

certification).

But this form of argument flies in the face of the rule

against nonparty preclusion. That rule perforce leads to

relitigation of many issues, as plaintiff after plaintiff after

plaintiff (none precluded by the last judgment because

none a party to the last suit) tries his hand at establishing

some legal principle or obtaining some grant of relief. We 

confronted a similar policy concern in Taylor, which involved litigation brought under the Freedom of Infor-

—————— 

11The great weight of scholarly authority—from the Restatement of

Judgments to the American Law Institute to Wright and Miller— 

agrees that an uncertified class action cannot bind proposed class 

members. See Restatement (Second) of Judgments §41(1), p. 393 (1980)

(A nonparty may be bound only when his interests are adequately

represented by “[t]he representative of a class of persons similarly

situated, designated as such with the approval of the court”); ALI,

Principles of the Law Aggregate Litigation §2.11, Reporters’ Notes, cmt. 

b, p. 181 (2010) (“[N]one of [the exceptions to the rule against nonparty

preclusion] extend generally to the situation of a would-be absent class

member with respect to a denial of class certification”); 18A Wright &

Miller §4455, at 457–458 (“[A]bsent certification there is no basis for 

precluding a nonparty” under the class-action exception). 
Cite as: 564 U. S. ____ (2011) 17 

Opinion of the Court 

mation Act (FOIA). The Government there cautioned 

that unless we bound nonparties a “‘potentially limitless’” 

number of plaintiffs, perhaps coordinating with each 

other, could “mount a series of repetitive lawsuits” demanding the selfsame documents. 553 U. S., at 903. But 

we rejected this argument, even though the payoff in a 

single successful FOIA suit—disclosure of documents to

the public—could “trum[p]” or “subsum[e]” all prior losses, 

just as a single successful class certification motion could

do. In re Bridgestone/Firestone, 333 F. 3d, at 766, 767. As 

that response suggests, our legal system generally relies 

on principles of stare decisis and comity among courts to 

mitigate the sometimes substantial costs of similar litigation brought by different plaintiffs. We have not thought

that the right approach (except in the discrete categories 

of cases we have recognized) lies in binding nonparties to a

judgment.

And to the extent class actions raise special problems of 

relitigation, Congress has provided a remedy that does not 

involve departing from the usual rules of preclusion. In 

the Class Action Fairness Act of 2005 (CAFA), 28 U. S. C.

§§1332(d), 1453 (2006 ed. and Supp. III), Congress enabled 

defendants to remove to federal court any sizable class 

action involving minimal diversity of citizenship. Once 

removal takes place, Federal Rule 23 governs certification.

And federal courts may consolidate multiple overlapping 

suits against a single defendant in one court (as the Judicial Panel on Multi-District Litigation did for the many

actions involving Baycol). See §1407. Finally, we would 

expect federal courts to apply principles of comity to each

other’s class certification decisions when addressing a

common dispute. See, e.g., Cortez Byrd Chips, Inc. v. Bill 

Harbert Constr. Co., 529 U. S. 193, 198 (2000) (citing 

Landis v. North American Co., 299 U. S. 248, 254 (1936)). 

CAFA may be cold comfort to Bayer with respect to suits 

like this one beginning before its enactment. But Con-
18 SMITH v. BAYER CORP. 

Opinion of the Court 

gress’s decision to address the relitigation concerns associated with class actions through the mechanism of removal

provides yet another reason for federal courts to adhere 

in this context to longstanding principles of preclusion.12 

And once again, that is especially so when the federal

court is deciding whether to go so far as to enjoin a state

proceeding. 

* * * 

The Anti-Injunction Act prohibits the order the District

Court entered here. The Act’s relitigation exception authorizes injunctions only when a former federal adjudication clearly precludes a state-court decision. As we said 

more than 40 years ago, and have consistently maintained

since that time, “[a]ny doubts . . . should be resolved in

favor of permitting the state courts to proceed.” Atlantic 

Coast Line, 398 U. S., at 297. Under this approach, close

cases have easy answers: The federal court should not

issue an injunction, and the state court should decide the 

preclusion question. But this case does not even strike us 

as close. The issues in the federal and state lawsuits 

differed because the relevant legal standards differed. 

And the mere proposal of a class in the federal action

could not bind persons who were not parties there. For 

these reasons, the judgment of the Court of Appeals is 

Reversed. 

—————— 

12By the same token, nothing in our holding today forecloses legislation to modify established principles of preclusion should Congress 

decide that CAFA does not sufficiently prevent relitigation of class

certification motions. Nor does this opinion at all address the permissibility of a change in the Federal Rules of Civil Procedure pertaining to

this question. Cf. n. 7, supra (declining to reach Smith’s due process

claim).