WASHINGTON (AP) _ The judge in the Iran-Contra conspiracy case is sticking with his decision to hold separate trials for former White House aide Oliver L. North and three co-defendants.

U.S. District Judge Gerhard A. Gesell on Tuesday rejected independent counsel Lawrence E. Walsh's suggestion that he try North and former national security adviser John M. Poindexter together before two juries.

Walsh's proposal for a dual-jury trial, followed by another for arms dealers Albert Hakim and retired Air Force Maj. Gen. Richard V. Secord, was made last week after Gesell ordered four separate trials.

The prosecutor, who suffered a tactical and logistical blow by the judge's ruling, contended that four separate trials would inconvenience both the prosecution and the defense.

Gesell ruled that each of the four men accused of conspiring to divert U.S.-Iran arms-sale profits to the Nicaraguan rebels must be tried separately to allow them to use each other's immunized congressional testimony as evidence.

A joint trial would be impossible, Gesell ruled, because a defendant's right not to have his congressional testimony used against him would conflict with a co-defendant's right to use all available evidence to help his defense.

The judge has not ruled definitively on whether the congressional immunity given North, Poindexter and Hakim would prevent the cases from going forward.

But Gesell gave further indication Tuesday that he planned to proceed with a trial of North, whom Walsh has designated as the lead-off defendant.

In a memorandum issued to explain why he canceled Monday's hearing to take testimony on the issue, Gesell said he wanted ''to defer this aspect for post- trial ... consideration, at which time counsel will be guided by the court's forthcoming ... opinion.''

The defense contends that the prosecution was tainted by exposure to the testimony the three men gave under grants of limited immunity from prosecution.

Gesell has indicated he will rule on pending pre-trial motions by July 1 to enable him to set a prompt trial date.

The judge rejected as too cumbersome Walsh's proposal for two-jury trials. Under Walsh's plan, the jury considering the evidence against Poindexter, for example, would have left the courtroom when North's lawyers cited the former national security adviser's congressional testimony.

''Quite unlike any prior two-jury trial, the need to exclude one jury alternating with the other will continually arise with practically every witness and an orderly trial free of error could not be achieved,'' Gesell said.

The judge also indicated that it might be unnecessary for Walsh to divide his staff into three trial teams to prevent his prosecutors from being tainted by exposure to congressional testimony cited during the trials.

''Counsel for all parties are advised that once the government is exposed to the immunized testimony during a trial of one defendant, the court is presently aware of no decision that would foreclose the same prosecuting attorneys handling the first trial from full participation in subsequent trials,'' the judge said. ''But resolution of this issue must await further developments.''