A prison inmate using a pseudonym to hide his identity began testifying in federal court today about his alleged involvement as an informant in one of a series of holdups that Prince George's County police are accused of staging to catch criminal suspects in 1967.

But the prisoner, who appeared distressed and jittery, said little in his brief appearance on the stand before trial Judge Herbert F. Murray adjourned for the day. Much of the earlier part of the day was taken up by attorneys seeking ways to arrange the prisoner's testimony without disclosing his identity. Meeting with Judge Murray away from the jury, the attorneys said the prisoner expressed fears of reprisals by fellow inmates.

In an unusual move, Murray at one point called two news reporters at the trial to the bench and asked if they would agree to a request by an attorney representing the prisoner to use only the prisoner's proposed pseudonym in news accounts and conceal the fact that the name was a pseudonym.

The two reporters, representing The Washington Post and The Washington Times, and later a third reporter representing The Baltimore Sun, consulted by telephone with their editors. They then returned to the bench conference and told Murray they would agree to use the pseudonym (or no name at all) and not disclose the prisoner's real name or whereabouts. But they said they could not agree to publishing the pseudonym as though it were the prisoner's real name and without giving an explanation for its use.

Attorneys for the prisoner and for several plaintiffs suing the police in the case agreed to both the pseudonym arrangement and its disclosure. Attorneys representing the police and Prince George's County government said they wished to dissociate themselves from any attempt to influence the press.

The jury was then reconvened and the prisoner was brought into the courtroom. Judge Murray distributed copies of a note to the six-member jury and six alternates explaining the prisoner's use of a pseudonym and the reason for it.

Attorneys for the plaintiffs in the $9 million civil rights case had told the jury during opening arguments several weeks ago that witnesses--including the prisoner using the pseudonym "Chaney"--would testify about a holdup of a 7-Eleven convenience store in Hyattsville on Nov. 24, 1967, that was allegedly staged by members of a so-called "death squad" of county police detectives. The attorneys said that "Chaney," then an informant, recruited suspect Pedro Gonzales at police request to participate in the robbery. Police waiting at the store shot and wounded Gonzales and let "Chaney" go, the attorneys said.

"Chaney" repeatedly told attorneys asking about activities leading to his involvement in the 1967 incident that he could not remember details. He is expected to return to the witness stand Wednesday.

Police defendants, including Joseph D. Vasco Jr., now the second-highest ranking official in the county police department, have denied allegations of staging a series of five holdups and burglaries.