A St. Mary's County jury found transient volunteer fireman William Joseph Parker guilty today of murdering and raping 13-year-old Elizabeth C. Archard last summer near Bowie.

The conviction on charges of first-degree felony murder and second-degree rape is the first to fall under Marylandhs nine-month-old death penalty law.

After consulting with his attorney, Parker waived his right to be sentenced by the jury.Judge Howard S. Chasanow said he will decide in about a month which of two possible sentences to impose: death in Maryland's gas chamber or life imprisonment.

The jury reached is verdict after 15 1/2 hours of deliberation, ending a six-day trial moved here because of massive pretrial publicity.

The jury members found that Parker had murdered Archard while committing a felony-second-degree rape. They convicted Parker, 28, on the related charge of the illegal use of a handgun. He was found innocent of first-degree rape, first-degree murder by premeditation and abduction.

After Parker told the judge his decision to waive his right to be sentenced by the jury, his attorney, Fred Warren Bennett, broke down and began crying. Parker put his face in his hands and trembled slightly.

Then, as he was led from the courtroom, Parker whirled toward Det. Earl W. Jones, who headed the murder investigation, and yelled, "You son of a bitch."

"When I went over to the jail and saw Parker right after the verdict, for the first time in nine months he broke down," Bennett said. "It didn't get to me then but it did about 10 minutes later. It has been an extremely emotional trial for both of us."

Parker was accused of picking Archard up about a mile from her Annapolish home last Aug. 28, driving her to a wooded area in Bowie, then raping her, and shooting her five times. He was arrested Sept. 14 after implicating himself in a written statement made to Prince George's County police after questioning.

Jury foreman Maria P. Miller said after the verdict that the jury had been divided 10 to 2 in favor of convicting on first-degree murder by premeditation and eventually compromised and decided on felony murder.

"It was definitely a compromise," she said. "We were divided first on whether he was fully given all his rights. The major consensus-11 to 1-was that he was, and that our responsibility was also to consider the evidence.

"After that, there was some very strong feeling on the part of two jurors that it was second-degree murder. The majority wanted first degree by premeditation. The felony murder conviction was a compromise which one (of the two holdout) juror suggested."

Miller also said that the possibility that Parker could receive the death sentence was something the jury considered. "We discussed it for awhile and it was always in the back of our minds, but we finally decided that our job was to decide guilt or innocence."

Miller said the deliberations were "an extremely emotional experience for all of us," and conceded that there had been a lot of shouting back and forth. "There was some question in our minds about the way the police treated him.But we finally agreed his rights hadn't been violated."

As the verdict was read, Parker, who showed almost no emotion throughout the trial, sat motionless, starting at the jury.He said later he had "no comment" on the verdict.

Archard's parents, Philip D. Hale and Barbara Hale, sitting in the second row of the courtroom, held hands and said nothing when Miller answered "guilty" to the charge of first-degree felony murder. The Hales, like Parker, refused to talk to reporters.

"I think they are just very glad that it's over," prosecutor Edmond B. O'Connell said. "They're very relieved because now they can try to put this behind them. I think they believe the verdict was fair."

The charge of felony murder required the jury to find that Parker killed Archard while committing a felony.

After the verdict, there was some confusion over whether Parker should face a death sentence because of an apparent ambiguity in the new law. The statute reads that the defendant can face death if the murder was committed while he was attempting "robbery, arson or rape or a sexual offense in the first degree."

The verdict came after an emotional trial in which O'Connell argued that Parker was guilty of first-degree murder and the related charges because "he shot Elizabeth Archard not once, not twice, not three times or four times, but five times. Even if he had been sleepwalking the first shot should have awakened him."

Bennett based his defense largely on the tactics used by police detectives before Parker implicated himself in the crime. Bennett claimed, and Parker testified, that he had been worn down by the police, we kept him a headquarters for more than seven hours and used three different interrogators.

Parker finally told police, orally and in writing, that he had picked Archard up about a mile from her home, driven to the wooded area, and had sex with her. Then, he said, she told him that she was 13 and was going to tell her father.

Parker said he "blacked out" at that point and when he came to, he was standing over Archard, gun in hand.

On Friday, after the jury began deliberations, Parker told reporters that the thought of a possible death sentence "really scares me."

Standing in the middle of his tiny cell in the St. Mary's County detention center underneath a spot where the word "hell" had been printed in black letters, Parker talked about facing death.

"I'm scared. Ihve though about it a lot in the last few weeks," he said, as reporters stood in a semicircle outside the cell. "I know that it is a possibility. It's the kind of thing nobody should have to face or deal with, but that's the way the charge is."

Parker also said that seven months in jail had changed him and perhaps made him more religious. "Anyone who spends seven months in jail changes," he said. "I may be more religious now but I came in here a Catholic and I still am a Catholic. I've talked to a priest a few times though."

The thought of being convicted on Easter weekend occurred to him, Parker said. "I'm aware that it's Good Friday," he said yesterday. "I'd rather be in church than here. I'd rather be anywhere other than behind bars."

The verdict came just as the exhausted attorneys and Chasanow were about to meet in the judge's chambers to discuss a possible mistrial. One of the bailiffs heard cheering and clapping coming from the jury room. Two minutes later he received a note saying, "We have reached a verdict."