Lawyers for Bernard Ebbers, the former chief executive of WorldCom Incorporated, will be allowed to question the star prosecution witness about marital infidelity, a judge ruled Tuesday.

The decision was a boost for the defense, which will try to raise questions about the credibility and character of the witness, former WorldCom chief financial officer Scott Sullivan.

District Judge Barbara Jones said the line of questioning was permissible because it spoke to “Mr. Sullivan’s character for truthfulness.” Prosecutors were trying to block the defense from raising the fidelity issue.

Lawyers did not elaborate in court on the matter, and a call to Mr. Sullivan’s lawyer was not immediately returned.

The ruling came one day before questionnaires were to be handed out to potential jurors in the trial of Mr. Ebbers, who is accused of orchestrating an $11 billion accounting fraud at WorldCom, which now operates as MCI Incorporated.

In a second win for Mr. Ebbers, the judge blocked the government from introducing evidence about conversations between Messrs. Ebbers and Sullivan as they watched congressional hearings into the collapse of energy giant Enron Corporation.

Prosecutor David Anders said the conversations proved that Mr. Ebbers knew accounting tricks at WorldCom were “criminal, as opposed to just something accountants do, just innocent stuff.”

But Erik Kitchen, a lawyer for Mr. Ebbers, said it meant nothing that Mr. Ebbers watched the Enron hearings.

“The accounting activities at Enron are completely different than those involved in this case,” he said.

Ms. Jones also blocked the government from asking some other World-Com executives who have pleaded guilty in the case about Enron-related conversations.

“I think it goes without saying that this trial is not about Enron,” the judge said.

Mr. Ebbers, 63, faces charges of fraud, conspiracy, and making false filings to federal regulators. The charges carry up to 85 years in federal prison. He has pleaded not guilty.

Jurors will be interviewed one by one starting Monday, and opening statements could begin by the middle of next week. The trial is expected to last four to eight weeks.

The judge also upheld her previous ruling refusing to grant immunity to Ronald Beaumont, WorldCom’s former chief operating officer, and another former company executive.

The defense wants both to testify because it claims those executives were much more closely involved with accounting details and still did not know about irregularities that were being ordered by Mr. Sullivan.

Part of Mr. Ebbers’s defense is that the CEO did not concern himself with intimate details of WorldCom’s accounting, leaving the job to Mr. Sullivan instead.