opinion: UNITED STATES v. MECHANIK et al.
  No. 84-1640.
  Argued December 2, 1985
  Decided February 25, 1986
  Rehnquist, J., delivered the opinion of the Court, in which BURGER, C. J., and White, Powell, and Stevens, JJ., joined. BurgeR, C. J., filed a concurring opinion, post, p. 73. O’Connor, J., filed an opinion concurring in the judgment, in which Brennan and Blackmun, JJ., joined, post, p. 73. Marshall, J., filed a dissenting opinion, post, p. 80.
  Bruce J. Rosen argued the cause for petitioners in Nos. 84-1700 and 84-1704 and respondents in No. 84-1640. With him on the briefs were Michael D. Graves and Stephen J. Rogers.
  Mark I. Levy argued the cause for the United States. With him on the brief were Acting Solicitor General Fried, Assistant Attorney General Trott, Deputy Solicitor General Frey, and Vincent L. Gambale.
  Together with No. 84-1700, Lill v. United States, and No. 84-1704, Mechanik v. United States, also on certiorari to the same court.
  Justice Rehnquist
  delivered the opinion of the Court.
  Federal Rule of Criminal Procedure 6(d) states that only-specified persons including “the witness under examination” may be present at a grand jury proceeding. In these cases, two Government witnesses testified in tandem before the grand jury, which indicted respondents and cross-petitioners (hereafter defendants) Mechanik and Lili for various drug-related offenses and conspiracy to commit such offenses. The Court of Appeals for the Fourth Circuit held that the simultaneous presence of these two witnesses violated Rule 6(d), and that even though the petit jury subsequently returned a verdict of guilty against defendants, the verdict must be set aside on any count that corresponds to a “tainted” portion of the indictment. We believe that the petit jury’s verdict of guilty beyond a reasonable doubt demonstrates a fortiori that there was probable cause to charge the defendants with the offenses for which they were convicted. Therefore, the convictions must stand despite the rule violation.
  A fairly detailed summary of the District Court proceedings will help to illustrate the nature and extent of our holding. A grand jury returned an indictment charging defendants with drug-related offenses and conspiracy. This indictment was concededly free from any claim of error. The grand jury then returned a superseding indictment in which the conspiracy charge was expanded. In support of this superseding indictment, the United States Attorney presented the testimony of two law enforcement agents who were sworn together and questioned in tandem before the grand jury.
  The defendants did not learn about this joint testimony until after trial began. Before trial, they filed an omnibus motion requesting, inter alia, the names of all the people who appeared before the grand jury. The Government responded that there were no unauthorized persons appearing before the grand jury, and the District Court denied the motion. Trial began in February 1980, and concluded in early July of the same year. During the second week of trial, one Jerry Rinehart, an agent of the Drug Enforcement Administration, testified as a Government witness. At the time of his testimony, the Government furnished the defendants with a portion of the transcript of his grand jury testimony as required by the Jencks Act, 18 U. S. C. § 3500. The transcript disclosed that Rinehart and his fellow agent, Randolph James, had testified in tandem before the grand jury.
  The defendants moved for dismissal of the indictment on the ground that the simultaneous presence of the two agents had violated Federal Rule of Criminal Procedure 6(d). Chief Judge Knapp, presiding over the trial, concluded that the presence and testimony of the two agents had not violated Rule 6(d), and he denied the motion. In May 1980, however, Chief Judge Knapp was unexpectedly hospitalized, and Judge Copenhaver took over as the trial judge. The defendants then moved for a rehearing of their motion to dismiss the indictment. Judge Copenhaver took the motion under advisement until the conclusion of trial.
  In August 1980, after the jury had returned its guilty verdict, Judge Copenhaver ruled upon and denied the defendants’ motion for dismissal of the indictment. 511 F. Supp. 50 (SD W. Va. 1980). He first decided, contrary to Chief Judge Knapp’s earlier ruling, that the joint testimony of Agents Rinehart and James did constitute a violation of Rule 6(d). Id., at 53-58. But he declined to set aside the defendants’ indictment and convictions because, on the basis of a comparison between the two indictments and the evidence on which the indictments rested, the violation of Rule 6(d) had not harmed the defendants. Id., at 58-61. He justified this conclusion with respect to the substantive counts on the ground that they were materially unchanged from the valid initial indictment to the superseding indictment. Id., at 58-59. With respect to the conspiracy count, which had been expanded by the superseding indictment, he justified his conclusion on the ground that the grand jury “had before it ample independent evidence [apart from the joint testimony] to support a probable cause finding of the charges.” Id., at 61. In light of these conclusions, Judge Copenhaver determined that a post-trial dismissal of the indictment would simply confer a windfall benefit on the defendants “who stand convicted after a three-month trial conducted at enormous expense to the United States and the defendants.” Ibid. The judge nevertheless undertook to ensure future compliance with the one-witness rule by directing the Government to keep the court advised concerning compliance with Rule 6(d) in future criminal cases. Ibid.
  A divided Court of Appeals reversed the conspiracy convictions, affirmed the others, and dismissed the conspiracy portion of the indictment. 735 F. 2d 136 (1984). It reasoned that the language of Rule 6(d) is so “plain and unequivocal in limiting who may appear before a grand jury,” id., at 139, that its transgression requires automatic reversal of any subsequent conviction regardless of the lack of prejudice. Id., at 139-140. But the court reversed only the conspiracy convictions because it found that the violation of Rule 6(d) tainted only the portion of the superseding indictment that related to them. Id., at 140. A divided en banc decision agreed. 756 F. 2d 994 (1985) (per curiam).
  We assume for the sake of argument that the simultaneous presence and testimony of the two Government witnesses before the grand jury violated Rule 6(d), and that the District Court would have been justified in dismissing portions of the indictment on that basis had there been actual prejudice and had the matter been called to its attention before the commencement of the trial. But although the defendants appear to have been reasonably diligent in attempting to discover any error at the grand jury proceeding, they did not acquire the transcript showing that the two agents had appeared jointly in the grand jury proceeding until the second week of trial. Nor is there any suggestion that the Government designedly withheld the information. When the defendants made their motion to dismiss the indictment based on the joint testimony, Chief Judge Knapp denied the motion because of his view that there had been no violation of Rule 6(d). Judge Copenhaver eventually issued a contrary ruling on the resubmitted motion, but not until after the long and costly trial had been brought to its conclusion. Although we do not believe that the defendants can be faulted for any lack of diligence, We nonetheless hold that the supervening jury verdict made reversal of the conviction and dismissal of the indictment inappropriate.
  Both the District Court and the Court of Appeals observed that Rule 6(d) was designed, in part, “to ensure that grand jurors, sitting without the direct supervision of a judge, are not subject to undue influence that may come with the presence of an unauthorized person.” 736 F. 2d, at 139. The Rule protects against the danger that a defendant will be required to defend against a charge for which there is no probable cause to believe him guilty. The error involving Rule 6(d) in these cases had the theoretical potential to affect the grand jury’s determination whether to indict these particular defendants for the offenses with which they were charged. But the petit jury’s subsequent guilty verdict means not only that there was probable cause to believe that the defendants were guilty as charged, but also that they are in fact guilty as charged beyond a reasonable doubt. Measured by the petit jury’s verdict, then, any error in the grand jury proceeding connected with the charging decision was harmless beyond a reasonable doubt.
  It might be argued in some literal sense that because the Rule was designed to protect against an erroneous charging decision by the grand jury, the indictment should not be compared to the evidence produced by the Government at trial, but to the evidence produced before the grand jury. But even if this argument were accepted, there is no simple way after the verdict to restore the defendant to the position in which he would have been had the indictment been dismissed before trial. He will already have suffered whatever inconvenience, expense, and opprobrium that a proper indictment may have spared him. In courtroom proceedings as elsewhere, “the moving finger writes; and, having writ, moves on.” Thus reversal of a conviction after a trial free from reversible error cannot restore to the defendant whatever benefit might have accrued to him from a trial on an indictment returned in conformity with Rule 6(d).
  We cannot accept the Court of Appeals’ view that a violation of Rule 6(d) requires automatic reversal of a subsequent conviction regardless of the lack of prejudice. Federal Rule of Criminal Procedure 52(a) provides that errors not affecting substantial rights shall be disregarded. We see no reason not to apply this provision to “errors, defects, irregularities, or variances” occurring before a grand jury just as we have applied it to such error occurring in the criminal trial itself. See United States v. Hasting, 461 U. S. 499, 509 (1983); Chapman v. California, 386 U. S. 18, 21-24 (1967); United States v. Lane, 474 U. S. 438 (1986).
  The reversal of a conviction entails substantial social costs: it forces jurors, witnesses, courts, the prosecution, and the defendants to expend further time, energy, and other resources to repeat a trial that has already once taken place; victims may be asked to relive their disturbing experiences. See Morris v. Slappy, 461 U. S. 1, 14 (1983). The “[passage of time, erosion of memory, and dispersion of witnesses may render retrial difficult, even impossible.” Engle v. Isaac, 456 U. S. 107, 127-128 (1982). Thus, while reversal “may, in theory, entitle the defendant only to retrial, in practice it may reward the accused with complete freedom from prosecution,” id., at 128, and thereby “cost society the right to punish admitted offenders.” Id., at 127. Even if a defendant is convicted in a second trial, the intervening delay may compromise society’s “interest in the prompt administration of justice,” United States v. Hasting, supra, at 509, and impede accomplishment of the objectives of deterrence and rehabilitation. These societal costs of reversal and retrial are an acceptable and often necessary consequence when an error in the first proceeding has deprived a defendant of a fair determination of the issue of guilt or innocence. But the balance of interest tips decidedly the other way when an error has had no effect on the outcome of the trial.
  We express no opinion as to what remedy may be appropriate for a violation of Rule 6(d) that has affected the grand jury’s charging decision and is brought to the attention of the trial court before the commencement of trial. We hold only that however diligent the defendants may have been in seeking to discover the basis for the claimed violation of Rule 6(d), the petit jury’s verdict rendered harmless any conceivable error in the charging decision that might have flowed from the violation. In such a case, the societal costs of retrial after a jury verdict of guilty are far too substantial to justify setting aside the verdict simply because of an error in the earlier grand jury proceedings. The judgment of the Court of Appeals is therefore reversed to the extent it set aside, the conspiracy convictions and dismissed the indictment, but is otherwise affirmed.
  It is so ordered.
  In Vasquez v. Hillery, 474 U. S. 254 (1986), the Court set aside a final judgment of conviction because of racial discrimination in the composition of the grand jury that indicted the defendant. It found this result to be compelled by precedent directly applicable to the special problem of racial discrimination. Id., at 260-262. It also reasoned that racial discrimination in the selection of grand jurors is so pernicious, and other remedies so impractical, that the remedy of automatic reversal was necessary as a prophylactic means of deterring grand jury discrimination in the future, id., at 262, and that one could presume that a discriminatorily selected grand jury would treat defendants of excluded races unfairly. Id., at 263-264.
  We think that these considerations have little force outside the context of racial discrimination in the composition of the grand jury. No long line of precedent requires the setting aside of a conviction based on a rule violation in the antecedent grand jury proceedings, and the societal interest in deterring this sort of error does not rise to the level of the interest in deterring racial discrimination. See, e. g., Gerstein v. Pugh, 420 U. S. 103, 119-123 (1975); Coleman v. Alabama, 399 U. S. 1, 10-11 (1970); Chapman v. California, 386 U. S. 18 (1967).
  The Government argues that it was improper to reverse the conspiracy convictions because “[a]n indictment returned by a legally constituted and unbiased grand jury, like an information drawn by the prosecutor, if valid on its face, is enough to call for trial of the charge on the merits.” Costello v. United States, 350 U. S. 359, 363 (1956). We need not reach this argument of the Government because of the narrower ground upon which we rest our decision.
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