A Philadelphia death-row inmate who has been on the state's execution list since 1987 committed suicide yesterday at a state prison in southwestern Pennsylvania, state police and his lawyer said today.

The body of William Tilley, 46, was found hanging in his cell about 6:15 a.m. by corrections officers in the death-row unit at the State Correctional Institution in Greene County, south of Pittsburgh, police said.

"William Tilley spent years on death row seeking redress. It is now apparent that he became despondent," his lawyer, Billy H. Nolas, a federal defender, wrote in court papers to be filed tomorrow seeking to end the appeal.

It was the second suicide in a week at the state prison and the ninth over the last year in the 46,000-inmate state prison system.

Department of Corrections spokeswoman Susan McNaughton said that between five and 10 inmates have committed suicide statewide every year over the last five years, for an average of about eight per year.

Tilley was sentenced to death in 1987 for shooting to death Philadelphia Police Officer Robert Daiss during a July 1985 burglary at his Northeast Philadelphia home.

The prosecutor in Tilley's murder trial contended that Tilley was caught burglarizing the home and deserved death because he had killed Daiss during a felony. Tilley contended that he had acted in self-defense after Daiss confronted him with a gun.

Nolas said that Tilley had won a new sentencing hearing and that an appeal was proceeding on whether he should get an entirely new trial.

Death-row inmate Mumia Abu-Jamal, who knew Tilley, said Tilley worked as a librarian at the prison and was always upbeat, though he had - like many inmates - been frustrated by the slow pace of appeals.

But Abu-Jamal said he had no idea what prompted the suicide. "No one saw it coming," he said in a telephone interview.

Tilley was one of 227 inmates awaiting execution in Pennsylvania, which has the fourth-largest death-row population in the nation.

In the court papers to be filed tomorrow with the state Supreme Court, Nolas made numerous references to studies and judicial comments about the effect of long-term confinement to death row.

He quoted the late Justice Felix Frankfurter as having said: "The onset of insanity while awaiting execution of a death sentence is not a rare phenomenon."