opinion: Mr. Justice Douglas
  delivered the opinion of the Court.
  Petitioner, a member of the Ohio bar, has been held in contempt of a state court and sentenced to jail and fined. He brought this habeas corpus proceeding in the state courts challenging, inter alia, their jurisdiction to punish him for the conduct in question. He was denied relief by the Supreme Court of Ohio on a divided vote, 172 Ohio St. 269, 175 N. E. 2d 59. We granted the petition for certiorari, 368 U. S. 894.
  The matter in dispute arose out of a management-labor controversy. The employer sought and obtained from the state court an injunction against picketing. Petitioner had been retained by the International Longshoremen's Association to represent the local involved in this labor dispute and when advised by the clerk of the court that a petition for an injunction had been requested, he said he would be ready any time for a hearing. The injunction, however, was issued ex parte. Petitioner immediately asked for a hearing; but none was granted. At the time the ex parte injunction issued, the union had on file with the National Labor Relations Board an unfair labor practice charge, but no hearing had been held on it.
  Petitioner, believing that under Ohio Rev. Code, 1954, § 2727.06 the restraining order was invalid because it was issued without a hearing and that the controversy was properly one for the National Labor Relations Board and not for the state court, advised,the union officials that the restraining order was invalid and that the best way to contest it was to continue picketing and, if the pickets were held in contempt, to appeal or to test any order of commitment by habeas corpus. The union officials agreed on that course of action and the picketing continued.
  Petitioner again sought to obtain a hearing on his motion to vacate the restraining order. But the judge said none could be had for a week. Thereupon petitioner and opposing counsel agreed to submit four pickets for a contempt hearing and to stipulate the facts.
  He produced the four pickets the following day and the judge held them in contempt, giving them two days to purge themselves. At the end of the two days another hearing was held; the pickets did not purge themselves. Petitioner made clear at this hearing and at the earlier one that it was he who had advised the union to test the injunction by risking contempt. The judge held him in contempt for disobeying or resisting “a lawful writ, process, order, rule, judgment, or command” of the court. Ohio Rev. Code, 1954, § 2705.02. While an opportunity was given petitioner to be heard, petitioner was not allowed to testify on his own behalf, the judge ruling that the only purpose of the hearing was to sentence petitioner.
  There was a hearing in the Ohio Court of Appeals when a petition for habeas corpus was filed; and at that hearing the undisputed facts showed that the employer was engaged in interstate commerce, that when the contempt order was issued an unfair labor practice charge involving the same dispute as the picketing was pending before the National Labor Relations Board, and that the picketing which had been enjoined was peaceful picketing.
  Respondent argues that the controversy between the employer and the union involved no attempt to organize workers and no refusal of the employer to bargain but only the enforcement of a “no-strike” clause in a collective bargaining agreement which was left by Congress either to federal courts (Textile Workers Union v. Lincoln Mills, 353 U. S. 448) or to state courts. Teamsters Local v. Lucas Flour Co., 369 U. S. 95.
  Petitioner, however, argues that the unfair labor practice charge filed with the National Labor Relations Board was based on the refusal of the employer to bargain in good faith and that the collective bargaining agreement which the employer asked the state court to enforce had been signed by unauthorized agents.
  We said in In re Oliver, 333 U. S. 257, 275, that procedural due process “requires that one charged with contempt of court be advised of the charges against him, have a reasonable opportunity to meet them by way of defense or explanation, have the right to be represented by counsel, and have a chance to testify and call other witnesses in his behalf, either by way of defense or explanation.”
  Petitioner was guilty of no misconduct that fell within the category of acts which constitute contempt in open court, where immediate punishment is necessary to prevent “demoralization of the court’s authority” (id., at 275) or the other types of contempt considered in Brown v. United States, 359 U. S. 41. The question was whether the state court was trenching on the federal domain. The issue thus tendered emphasizes one important function that a hearing performs. It is impossible to determine from this record whether or not the dispute was exclusively within the jurisdiction of the National Labor Relations Board under the principles of San Diego Building Trades Council v. Garmon, 359 U. S. 236, and Amalgamated Association v. Wisconsin Employment Relations Board, 340 U. S. 383. The Ohio court could not know whether it was within bounds in citing a person for contempt for violating the injunction without such a hearing. For, as Amalgamated Association v. Wisconsin Employment Relations Board, supra, held, a state court is without power to hold one in contempt for violating an injunction that the state court had no power to enter by reason of federal pre-emption. Even if we assume that an ex parte order could properly issue as a matter of state law, it violates the due process requirements of the Fourteenth Amendment to convict a person of a contempt of this nature without a hearing and an opportunity to establish that the state court was acting in a field reserved exclusively by Congress for the federal agency. When an activity is “arguably” subject to the National Board the States must defer to its “exclusive competence,” “if the danger of state interference with national policy is to be averted.” San Diego Building Trades Council v. Garmon, supra, at 245.
  Reversed
  Mr. Justice Frankfurter and Mr. Justice White took no part in the consideration or decision of this case.
  One of the companion cases in Amalgamated Association v. Wisconsin Employment Relations Board, supra, was No. 438, United Gas Workers v. Wisconsin Employment Relations Board, in which a conviction for contempt for not obeying a restraining order issued by the state court (258 Wis. 1, 44 N. W. 2d 547) was reversed. 340 U. S. 383, 386, 399. The opinion was written by Chief Justice Vinson who also wrote the opinion in United States v. United Mine Workers, 330 U. S. 258. The latter case involved a restraining order of a federal court and presented no question of pre-emption of a field by Congress where, if the federal policy is to prevail, federal power must be complete.
  Cf. Ex parte Bradley, 7 Wall. 364, 375; Hovey v. Elliott, 167 U. S. 409.
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