Cmdr. Donal M. Billig, the former chief heart surgeon at Bethesda Naval Hospital, was found guilty yesterday of involuntary manslaughter in the deaths of two patients and the negligent homicide of another in 1984. He was acquitted in the deaths of two other patients.

Billig, 55, was found guilty of two counts of involuntary manslaughter in the deaths of retired Lt. Col. John L. Kas Jr. of Cape Cod, and retired Petty Officer Joe Estep of Temple Hills. In the death of retired Maj. William Frank Grubb of Lancaster, S.C., the jury convicted Billig of a lesser charge, negligent homicide.

In only the second such court-martial of a Navy doctor, Billig also was convicted of 19 of 24 counts of dereliction of duty for operations in 1983 that the prosecution charged he undertook without proper supervision at the Navy's premier medical institution.

In two other cases -- the deaths of Lt. Col. Harold Coplan of Gaithersburg and Lois Parent, the wife of a Marine sergeant of Havelock, N.C. -- Billig was acquitted by a jury of nine Navy officers who listened to seven weeks of testimony.

The decision yesterday comes after a year of investigation by the Navy into the practices of the doctor and press reports and congressional hearings that have highlighted abuses in the military medical care system.

Under the conviction, Billig faces a maximum prison term of 11 years and nine months, dismissal from the Navy and fines.

The jury, which needed only a two-thirds consensus under military law, returned the decision late yesterday afternoon after more than 10 hours of deliberation in the last two days.

Billig stood and stared straight ahead as the president of the jury, Rear Adm. Harry S. Quast, read the verdict. He would not comment after the decision and left the courtroom holding the hand of his wife Bonnie. Defense counsel Lt. Cmdr. Stephen Baker said later he was disappointed with the verdict.

Sentencing procedures will begin next week and, under military law, the same jury will sentence Billig. Two-thirds of the panel has to agree to the sentence, if it is less than 10 years in prison. Three-quarters would have to agree to any term longer than 10 years.

Jury members refused to comment yesterday and prosecutors said they would not elaborate on the verdict until after sentencing. "I think it's just," Lt. Cmdr. Michael Curreri, one of three attorneys on the prosecution team, said seconds after the verdict.

Billig talked to reporters outside the courtroom as he waited for the decision and later, according to his attorney, even slept for 10 minutes inside legal counsel offices on the second floor of the building where the court-martial took place at the Washington Navy Yard. The surgeon expressed his confidence in military medicine and said he believed he "ran a service for Bethesda that was very acceptable."

"I don't think I should be here," he said, emphasizing he believed his mortality rate was comparable to the national average. Billig added then that he was pleased with his Navy defense lawyers, who argued the case before a panel of eight captains and one rear admiral. "You can't ask them to do more than what they could do," he said. "Nobody could do better under the circumstances."

The court-martial, which began Jan. 6, was a trial filled with technical explanations about the workings of the heart, statements from dozens of doctors, nurses and officers who had worked with Billig, teary testimony from relatives of the dead and demonstrations of surgical techniques rarely seen in a courtroom.

Unlike a civilian court, the military proceeding allows jurors to ask questions in written form. The members, including three doctors and a nurse, often submitted queries that focused on apparent inconsistencies on the medical conditions of the patients.

The prosecution brought in preserved heart specimens for jurors to examine early in the trial and then arranged for hearts of two of the patients who died -- Coplan and Grubb -- to be examined. Billig took the witness stand for five days and demonstrated his skill in suturing, which prosecutors had claimed was hindered by poor eyesight, as he sewed for 10 minutes before the jury.

The trial offered a rare glimpse of the inner practices of the military and Bethesda Naval Hospital. The prosecutors claimed Billig was an incompetent surgeon who had lost surgical privileges in the private sector and entered the Navy by lying about his past and an eye injury that had left him with little depth perception. The defense argued that Billig had been truthful about his limitations and was an honest surgeon who had become a scapegoat.

Billig arrived at Bethesda in 1983, two years after he had his privileges revoked at a New Jersey hospital that questioned his competence. He maintained throughout the trial that he had told recruiters about the problem in 1982 and was later told by them that the problems at Monmouth Medical Center in Long Branch, N.J., would not eliminate him from the Navy.

When and whether Bethesda officials knew about the problem was a point of contention throughout the trial. What is known is that Billig was appointed to the heart surgery unit with the condition that he would undergo a retraining program. He had told Bethesda officials that he had not practiced heart surgery for six years; they testified that they believed Billig could regain his skills with a program of practice and study.

It became evident from the testimony that Bethesda welcomed the new surgeon. Several heart surgeons were leaving the staff and Bethesda, a training hospital, needed a board-certified thoracic surgeon to head its heart program. Billig was made that program head in June 1983.

From June through September, Billig began undertaking operations on his own, the prosecution charged, although he was cautioned he still needed supervision. Billig and his defense attorney contended that the 24 operations he performed during those months were done with hospital knowledge.

The prosecution charged that Billig, who was suspended in November 1984, was hampered by poor eyesight from a tennis injury and should never have operated on any patients in 1983 and 1984. The five who died, they said, were killed when the doctor tore aortas and improperly sewed too deep or shallow during delicate surgeries.