A D.C. Court of Appeals ordered a new trial yesterday for a 32-year-old District man who argued successfully that he was convicted of sexually assaulting a minor in 1976 because his lawyer was incompetent.

The three-judge appellate panel ruled that Jimmy Johnson's "right to effective counsel" was ignored during his trial when his attorney, O. B. Parker, failed to call as a defense witness the physician whose medical testimoney could have contradicted an 11-year-old female's contention that she had been sexually assaulted.

The appeals court reversed Johnson's conviction and ordered a new trial after concluding that the defendant "has borne his heavy burden of showing gross incompetence by his trial counsel."

Johnson, who was sentenced to 18 months to 54 months in prison, was charged in July 1976 with taking indecent liberties with a minor. On June 6, 1976, an 11-year-old girl alleged, Johnson lured her to his apartment and assaulted her sexually.

At trial, Johnson -- the only witness who appeared in his defense -- denied that he assaulted the girl and insisted that she fabricated the story out of spite.

A doctor, who examined the girl at D.C. General Hospital shortly after the alleged attack, concluded in reports included in appellate court records that there was "no evidence of trauma in the complaintant's vaginal or genital area," the appellate judges wrote.

Parker was informed of the doctor's findings 12 days before trial, but failed to call the physician as a witness, the appeals court found. "The failure to develop the available medical testimoney 'blotted out the essence of a substantial defense . . . ,'" the panel concluded.