WASHINGTON (AP) _ Five months after Dr. Donal Billig joined the staff of Bethesda Naval Hospital to do heart surgery, a superior described his ability as being ''at the level of a first-year resident,'' a witness testified today.

Retired Navy Capt. Robert C. Cochran, who was responsible for supervising Billig, said the evaluation was made by Col. Ross Zajtchuk, an Army surgeon who was in charge of Billig's retraining in bypass surgery.

Billig had not done any heart surgery for seven years when he joined the Navy. He was accepted into the service in December 1982 and reported to the Bethesda hospital a month later.

In the ensuing two years, the Navy is charging at his court martial, he killed five patients through negligence and was derelict in his duties because he operated without supervision on 24 others.

Cochran, of Charleston, W.Va., testified that in the meeting on June 9, 1983 - after Zajtchuk expressed his low opinion of Billig's abilities - the decision was made that Billig should continue under observation and that he not be permitted to operate without a fully qualified surgeon present with him.

And because the Navy was short of heart surgeons, Zajtchuk and other surgeons from Walter Reed Army Hospital were to do the supervising.

Another witness, Capt. Judy Schwartz, has testified that after that meeting she wrote a recommendation that Billig become chief of cardiothoracic surgery even though it was understood, but not set down on paper, that he needed supervision for operations.

Cochran said that the concern at the meeting was that Bethesda could not attract surgical residents if it did not have a department head. Billig apparently used that recommendation to apply for full privileges, which he received in September.

Another witness, Dr. Manfred Cohen of Pittsburgh, said Billig was hired by his private firm of surgeons in the fall of 1981 but was let go after seven months.

''We felt Dr. Billig was not fitting into our practice,'' Cohen said. He alienated some of our physicians, and he was curt with the families of patients. It was more a matter of personality than of competence.''

Billig did no heart surgery in Pittsburgh, Cohen said.

Cochran, now retired from the Navy and in private practice in Charleston, was given a letter of censure last August from Secretary of the Navy John Lehman.

Scheduled to testify later today was Sandra Goetz, a secretary at the cardiothoracic department at Bethesda.