delivered the opinion of the Court.

In San Diego Building Trades Council v. Garmon, 359 U. S. 236, this Court held that the proper administration of the federal labor law requires state courts to relinquish jurisdiction not only over those controversies actually found to be within the jurisdiction of the National Labor Relations Board, but also over litigation arising from activities which might arguably be subject to that agency’s cognizance. Only such a rule, the Court held, will preserve for the Labor Board its congressionally delegated function of deciding what is and what is not within its domain.1 In the present case the Supreme Court of Minnesota held that the petitioners, Marine Engineers Beneficial Association (MEBA) and its Local 101, were not “labor organizations” within the meaning of § 8 (b) of the Labor Management Relations Act, 29 U. S. C. § 158 (b), and therefore not subject to the unfair labor practice provisions of that section of the statute. Accordingly, the court held that a state trial court had not erred in assuming jurisdiction over a labor dispute involving MEBA and Local 101, and in permanently enjoining them *175from picketing found to be in violation of state law. 260 Minn. 1, 108 N. W. 2d 627. We granted certiorari, 368 U. S. 811, to consider an asserted conflict between the Minnesota court’s decision and our holding in the Garmon case.

The essential facts which gave rise to this controversy are not in dispute. The respondents owned and operated a fleet of bulk cargo vessels on the Great Lakes. MEBA and Local 101 were, unions which represented marine engineers employed on the Great Lakes and elsewhere.2 The marine engineers employed by the respondents were not represented by MEBA or any other union.

On November 11, 1959, the respondents’ vessel, Samuel Mather, arrived at the dock of the Carnegie Dock and Fuel Company in Duluth, Minnesota. The following morning several members of Local 101 began to picket at the only entrance road to the Carnegie dock. They carried signs which read: “Pickands Mather Unfair to Organized Labor. This Dispute Only Involves P-M. M. E. B. A. Loe. 101 AFL-CIO.” and “M. E. B. A. Loe. 101. AFL-CIO. Request P-M Engineers to Join with Organized Labor to Better Working Conditions. This Dispute Only Involves P-M.” When the pickets appeared, employees of the Carnegie Dock and Fuel Company refused to continue unloading the Samuel Mather. As a result, the ship was forced to remain at the dock, and another of the respondents’ steamers, the Pickands, was compelled to ride at anchor outside the harbor for a number of days, because the Carnegie dock could accommodate but one vessel at a time.

*176Upon learning of the picket line, the respondents filed a complaint in the state court charging the union with several violations of state law. The complaint alleged, among other things, that the petitioners had induced Carnegie’s employees to refuse to perform services, and that the petitioners had thus caused Carnegie to breach its contract with the respondents. The petitioners filed a motion to dismiss the complaint, claiming that the dispute was arguably subject to the jurisdiction of the National Labor Relations Board and thus, under the Garmon doctrine, beyond the state court’s cognizance.3 Evidence was taken concerning the nature and effect of the picketing, the employment status of respondents’ marine engineers, and, to a limited extent, the characteristics of MEBA and Local 101. The trial court concluded that the dispute was within its jurisdiction, and, finding the picketing to be in violation of Minnesota law, it issued a temporary injunction prohibiting the petitioners from picketing at or near any site where the respondents’ vessels were loading or unloading, from inducing other employees or other firms not to perform services for the respondents, and from interfering in other specified ways with the respondents’ operations. The injunction was later made permanent on the basis of the same record, and the court's judgment was affirmed on review by the Supreme Court of Minnesota.

The Garmon case dealt with rules of conduct — whether certain activities were protected by § 7 or prohibited by § 8 of the Act. In the present case it has hardly been disputed, nor could it be, that the petitioners’ conduct was of a kind arguably prohibited by § 8 (b) (4) (A) of the Act and thus within the primary jurisdiction of the Board, if MEBA and Local 101 were “labor organizations” *177within the contemplation of § 8 (b) generally.4 The Minnesota courts determined, however, that those whom the petitioners represented and sought to enlist were “supervisors,” that consequently neither of the petitioners was a “labor organization,” and therefore that nothing in the Garmon doctrine precluded a state court from assuming jurisdiction.

It is the petitioners’ contention that the issue to be determined in this case is not whether the state courts correctly decided their “labor organization” status, but whether the state courts were free to finally decide that issue at all. The petitioners contend that the principles of the Garmon decision confined the state court to deciding *178only whether the evidence in this case was sufficient to show that either of them was arguably a “labor organization” within the contemplation of § 8 (b). We agree, and hold that the evidence was sufficient to deprive the Minnesota courts of jurisdiction over this controversy.

We see no reason to assume that the task of interpreting and applying the statutory definition of a “labor organization” does not call for the same adjudicatory expertise that the Board must bring to bear when it determines the applicability of §§ 7 and 8 of the Act to substantive conduct. Indeed, analysis of the problem makes clear that the process of defining the term “labor organization” is one which may often require the full range of Board competence.

The term “labor organization” is defined by § 2 (5) of. the Act, which says:

“The term ‘labor organization' means any organization of any kind, or any agency or employee representation committee or plan, in which employees participate and which exists for the purpose, in whole or in part, of dealing with employers concerning grievances, labor disputes, wages, rates of pay, hours of employment, or conditions of work.” 29 U. S. C. § 152 (5).

The part of that definition at issue in the present case is the requirement that “employees participate” in the organization. As defined by § 2 (3) of the Act, “[t]he term ‘employee' . . . shall not include . . . any individual employed as a supervisor ....'' 29 U. S. C. § 152 (3).5 “Supervisor” is defined in turn by § 2 (11) of the Act to mean:

“. . . any individual having authority, in the interest of the employer, to hire, transfer, suspend, lay off, *179recall, promote, discharge, assign, reward, or discipline other employees, or responsibly to direct them, or to adjust their grievances, or effectively to recommend such action, if in connection with the foregoing the exercise of such authority is not of a merely routine or clerical nature, but requires the use of independent judgment.” 29 U. S. C. § 152 (11).

The statutory definition of the term “supervisor” has been the subject of considerable litigation before the NLRB and in the federal courts.6 It is immediately apparent, moreover, that the phrase “organization ... in which employees participate” is far from self-explanatory. Several recurring questions stem from the fact that na*180tional or even local unions may represent both “employees” and “supervisors.” 7 For example, is employee participation in any part of a defendant national or local union sufficient, or must “employees” be involved in the immediate labor dispute?8 What percentage or degree of employee participation in the relevant unit is required? 9 If an organization is open to “employees” or solicits their membership, must there be a showing that there are actually employee members? And, if a local union is not itself a “labor organization,” are there conditions under which it may become subject to § 8 (b) as an agent of some other organization which is? 10

The considerations involved in answering these questions are largely of a kind most wisely entrusted initially to the agency charged with the day-to-day administration of the Act as a whole. The term “labor organization” appears in a number of sections of the Act. Section 8 (a) (2), for example, forbids employers to “dominate or interfere with the formation or administration of any labor organization or contribute financial or other support to it . . . .” 29 U. S. C. § 158 (a) (2). Section 8 (a) (3) makes it an unfair labor practice for an employer, by certain discriminatory conduct, “to encourage or discourage membership in any labor organization . . . .” 29 U. S. C. §158 (a)(3). Section 9(c), dealing with the largely *181unreviewable area of representation elections,11 refers repeatedly to both “employees” and “labor organizations.” The policy considerations underlying these and other sections of the Act, and the relationship of a particular definitional approach under § 8 (b) to the meaning of the same term in the various sections, must obviously be taken into account if the statute is to operate as a coherent whole.12 A centralized adjudicatory process is also essential in working out a consistent approach to the status of the many separate unions which may represent interrelated occupations in a single industry.13 Moreover, as the national agency charged with the administration of federal labor law, the Board should be free in the first instance to consider the whole spectrum of possible approaches to the question, ranging from a broad definition of “labor organization” in terms of an entire union to a narrow case-by-case consideration of the issue. Only the Board can knowledgeably weigh the effect of either choice upon the certainty and predictability of labor management relations, or assess the importance of simple administrative convenience in this area.14

*182For these reasons we conclude that the task of determining what is a “labor organization” in the context of § 8 (b) must in any doubtful case begin with the National Labor Relations Board, and that the only workable way to assure this result is for the courts to concede that a union is a “labor organization” for § 8 (b) purposes whenever a reasonably arguable case is made to that effect. Such a case was made in the Minnesota courts.

There persuasive evidence was introduced to show that all the marine engineers employed by the respondents were in fact supervisors.15 It was also shown that MEBA had steadfastly maintained in proceedings before the NLRB that it was not a labor organization subject to § 8 (b) of the Act.16 However, the petitioners introduced into the record two recent Board decisions, one holding *183that MEBA was subject to § 8 (b) and was guilty of an unfair labor practice for engaging in an activity similar to that involved in this case,17 and the other holding that marine engineers represented by a branch of Local 101 were “employees” for the purpose of a § 9 (c) election.18 The Board’s order in the first case was enforced by the Court of Appeals for the Second Circuit on January 13, 1960, during the pendency of the present litigation in the Minnesota trial court.19 The state court’s attention was expressly called to the Board’s theory, subsequently adopted by the Court of Appeals for the Second Circuit, that the relevant unit of membership for determining what is a labor organization in a § 8 (b) context is the entire union, and to the holding that the known membership of a few “employees,” provisions in the union’s constitution making membership available to “employees,” and previous conduct indicative of “employee” representation were sufficient to render the national union a “labor organization.” See 121 N. L. R. B., at 209-210'; 274 F. 2d, at 174 — 175. Three additional District Court decisions expressly holding that the Board had reasonable cause to believe that MEBA or Local 101 was subject to § 8 (b) had been decided before the issuance of the Minnesota trial court’s judgment in the present litigation, although the record does not show that these were specifically brought to the court’s attention.20

*184This was a case, therefore, where a state court was shown not simply the arguable possibility of Labor Board jurisdiction over the controversy before' it, but that the Board had actually determined the underlying issue upon which its jurisdiction depended, i. e., that MEBA was a “labor organization” for purposes of § 8 (b) of the Act. In the absence of a showing that this position had been authoritatively rejected by the courts,21 or abandoned by the Labor Board itself, we hold that it was the duty of the state court to defer to the Board’s determination.22

*185The need for protecting the exclusivity of NLRB jurisdiction is obviously greatest when the precise issue brought before a court is in the process of litigation through procedures originating in the Board. While the Board’s decision is not the last word, it must assuredly be the first. In addition, when the Board has actually undertaken to decide an issue, relitigation in a state court creates more than theoretical danger of actual conflict between state and federal regulation of the same controversy.23 “Our concern” here, as it was in the Garmon case, 359 U. S., at 246, “is with delimiting areas . . . which must be free from state regulation if national policy is to be left unhampered.”

Reversed.

dissenting.

While I agree with the principles announced by the Court, I disagree with the result that is reached on the facts of this case. The record contains an affidavit of the President of this union, the Marine Engineers Beneficial Association (MEBA), which states that all members of the union,' including the local involved in this case, perform supervisory functions.1

*186An officer of MEBA testified:

“Local 101 of the Marine Engineers Beneficial Association is comprised of those men who are licensed as marine engineers by the United States Coast Guard, and those men who perform the engineering duties of engineers, whether or not they are licensed by the Coast Guard.”

The record makes clear that a licensed engineer has supervisory duties whenever there is someone working under him. That status is grounded in the historic distinction between licensed and unlicensed personnel and is shown by this record.2 A union of masters and mates *187would plainly be a union of supervisors and under present law not be qualified to represent ordinary seamen. If there are rare instances when an engineer on a tug, for example, is nothing more than an employee, that has not been shown in the record and is directly contrary to the affidavit of this union’s president.

The trial court in this case said that the record “does not show” that this MEBA Local “admits to membership any non-supervisory employee, and in any event it is clear that its membership is composed primarily and almost exclusively of supervisors.” That finding is not challenged here. Petitioners, placing all their hopes on the words of the trial court that this local is composed “primarily and almost exclusively of supervisors,” say it may therefore be arguably and reasonably contended that the local is a labor organization within the meaning of the Act.

Section 2 (5) defines “labor organization” as any organization “in which employees participate” for the purpose “of dealing with employers concerning grievances,” etc.

The word “employee” was redefined by Congress3 following our decision in Packard Co. v. Labor Board, 330 U. S. 485, so as to exclude “any individual employed as a supervisor.” § 2 (3). And § 14 (a) provides that “Nothing herein shall prohibit any individual employed as a supervisor from becoming or remaining a member of a labor organization, but no employer subject to this Act shall be compelled to deem individuals defined herein as supervisors as employees for the purpose of any law, either national or local, relating to collective bargaining.”

*188There is not a shred of evidence in this record showing that any employee not a supervisor is a member of this union. There is therefore not a shred of evidence to show that this local of MEBA is a “labor organization.” Since there is not, it has made no showing that it is entitled to any of the protections of the Federal Act. Such a showing is within its power to make. It apparently claims to be a “labor organization” when it is to its advantage to do so and protests against being so labeled when that position serves its end.4

If it desires the protection of the Federal Act, it should be required to come forth with evidence showing who its members are. In absence of such a showing, we should not disturb the rulings of the Minnesota courts, which on this record were fully justified in enjoining the picketing. It wras indeed conceded by counsel for MEBA at the trial that the purpose of the picketing was “to improve the wages, hours and working conditions” of the “licensed engineers,” not the wages, hours and working conditions of those few undisclosed individuals who it is now intimated may have been members of the union.

Since this local is not on this record a “labor organization,” it does not come within the purview of § 8 (b) (2) or § 8 (b)(4), which makes certain practices, alleged to have taken place here, unfair labor practices. For § 14, quoted above, returned supervisors to the basis which they enjoyed prior to the Federal Act. Bull S. S. Co. v. National Marine Eng. B. Assn., 250 F. 2d 332.

*189It matters not that at other times this local or MEBA may have been a “labor organization” for purposes of the Federal Act.5 Apparently an engineer may at times be only an ordinary employee.6 So for one operation this local may have members doing the work of nonsupervisory employees. Whether its status would therefore change from day-to-day or week-to-week might be presented in some case. It is not presented here, for, on a record showing only supervisors among the membership list, the union has no claim to shelter under the Federal Act.