delivered the opinion of the Court.
Pеtitioner Thomas West brought a “hybrid” suit against his employer, his union, and his union representative under the Railway Labor Act. He alleged that the employer had breached the collective-bargaining agreement and that the union and its representative had breached their duty of fair representation. The parties agree, for the purpose of our review of the Court of Appeals’ judgment, that petitioner’s cаuse of action accrued on March 25,1984, the date petitioner learned of the alleged breach of the union’s duty of fair representation. His complaint was filed on September 24, 1984, less than six months after the statute of limitations began to run. The summonses and complaints were mailed to respondents on October 10, 1984. Respondents acknowledged service of the complaint on dates ranging from October 12, 1984, thrоugh November 1, 1984. Thus, both the date on which the complaints were mailed and the date when the first acknowledgment of service was made were more than six months after the statute began to run.
Because service was not effected within the 6-month period prescribed in § 10(b) of the National Labor Relations Act,
1
*37
the District Court granted respondents’ motion for summary judgment. App. to Pet. for Cert. 15a. The Court of Appeals for the Third Cirсuit affirmed.
Congress did not enact a federal statute of limitations that is expressly applicable to federаl duty of fair representation claims. In
DelCostello
v.
Teamsters,
The only gap in federal law that we intended to fill in DelCostello was the appropriate limitations period. We did not intend to replace any part of the Fеderal Rules of Civil Procedure with any part of § 10(b) of the National Labor Relations Act. Rule 3 of the Federal Rules of Civil Procedure provides that a civil action is commenced by filing a complaint with the cоurt, and Rule 4 governs the procedure for effecting service and the period within which service must be made. The clerk of the district court must “forthwith issue a *39 summons and deliver the summons to the plaintiff or the plaintiff’s attorney, who shall be responsible for prompt service of the summons and a copy of the complaint.” Fed. Rule Civ. Proc. 4(a). Service must normally be made within 120 days. See Rule 4(j). Although we have not expressly so held before, we now hold that when the underlying cause of action is based on federal law and the absence of an express federal statute of limitations makes it necessary to borrow a limitations period from another statute, the action is not barred if it has been “commenced” in compliance with Rule 3 within the borrowed period. 4 See 4 C. Wright & A. Miller, Federal Practice and Procedure § 1056 (1969). We decline respondents’ invitatiоn to require that when a federal court borrows a statute of limitations to apply to a federal cause of action, the statute of limitation’s provisions for service must necessarily also be followed, even when the borrowed statute is to be applied in a context somewhat different from the one in which those procedural rules originated. 5
Inevitably our resolution of cases or controversiеs requires us to close interstices in federal law from time to time, but when it is necessary for us to borrow a statute of limitations for a federal cause of action, we borrow no more than neces *40 sary. 6 Here, bеcause of the availability of Rule 3, there is no lacuna as to whether the action was brought within the borrowed limitations period. 7
The judgment of the Court of Appeals is reversed, and the case is remanded fоr further proceedings consistent with this opinion.
It is so ordered.
Notes
Section 10(b) of the National Labor Relations Act, 49 Stat. 453, as amended, 29 U. S. C. § 160(b), provides:
*37 “Whenever it is charged that any person has engaged in or is engaging in any such unfair labor practice, the Board, or any agent or agency designated by the Board for such purposes, shall have power to issue and cause to be served upon such person a complaint stating thе charges in that respect, and containing a notice of hearing before the Board or a member thereof, or before a designated agent or agency, at a place therein fixed, not lеss than five days after the serving of said complaint: Provided, That no complaint shall issue based upon any unfair labor practice occurring more than six months prior to the filing of the charge with the Board and the servicе of a copy thereof upon the person against whom such charge is made, unless the person aggrieved thereby was prevented from filing such charge by reason of service in the armed forces, in which event the six-month period shall be computed from the day of his discharge. Any such complaint may be amended by the member, agent, or agency conducting the hearing or the Board in its discretion at any time prior to the issuance of an order based thereon. The person so complained of shall have the right to file an answer to the original or amended complaint and to appear in person or otherwise and give testimony at the place and time fixed in the complaint. In the discretion of the member, agent, or agency conducting the hearing or the Board, any other person may be allowed to intervene in the said proceeding and to present testimony. Any such proceeding shall, so far as practicable, be conducted in accordance with the rules of evidence applicable in the district courts of the United States under the rules of civil procedure for the district courts of the United States, adopted by the Supreme Court of the United States pursuant to section 2072 of title 28.” (Emphasis added.)
Although
DelCostello
and the Sixth Circuit’s opinion in
Macon
v.
ITT Continental Baking Co.,
Under § 10(b), the еmployee’s charge is timely if a copy is served personally or mailed within the limitations period. See 29 CFR § 102.113(a) (1986). The complaint in an unfair labor practice proceeding is filed by the General Counsel after he or she has investigated the employee’s charge. See 29 U. S. C. § 153(d).
When the underlying cause of action is based on state law, and federal jurisdiction is based on diversity of citizenship, state law not only provides the appropriate period of limitations but also determines whether service must be effected within that period.
Walker
v.
Armco Steel Corp.,
Our holding that the statute of limitations was tolled when the complaint was filed eliminates the potential difficulty of determining the actual dates on which serviсe of the complaint was made on the various defendants.
In some cases, the determination of the length of the borrowed period may require examination of the tolling rules that are followed in the jurisdiсtion from which the statute of limitations is borrowed. See,
e. g., Wilson
v.
Garcia,
Respondents also argue that §10(b)’s service requirement must be adoptеd in order to assure that defendants receive prompt notice of suit against them. The requirement of timely service in Rule 4(j) satisfies this need without recourse to the service requirement of § 10(b). While it is possible that а defendant Will not be served with the complaint until 10 months after the cause of action accrues, this result is not inconsistent with our adoption of a 6-month statute of limitations for breach of contract/ breach of duty of fair representation claims. See
DelCostello
v.
Teamsters,
