John Watson Lewis, 29, an assistant professor of history at Bowie State College whose body was found Sunday in his Washington apartment, died of a gunshot wound to the head, the office of the D.C. medical officer said Thursday.

The death was listed as a homicide. Detectives said the motive for the slaying is under investigation. There were no suspects in custody, police said Thursday.

Mr. Lewis started teaching at Bowie last fall. Before moving there, he had taught Afro-American history at Case Western Reserve University in Cleveland. He also had taught history education at Niagara Community College in New York and history at Lincoln University in Pennsylvania.

He was born in Washington. A graduate of Cardozo High School, he earned a bachelor's degree from Lincoln University and a master's degree from the State University of New York at Fredonia. He took further studies at the University of Buffalo, the University of California at Irvine, and case Western Reserve.

Mr. Lewis had been organist at the Greater First Baptist Church in Washington. He was associated with the National Endowment of the Humanities, and belonged to the NAACP.

He is survived by his mother, Mabel L. Lewis, a brother, Reid A. Jr., and a sister, Sheila D., all of Washington; another sister, Carolyn V. Guest, of New Carrollton, and his grandmother, Mae Belle Lee, of Aulander, N.C. CAPTION: Picture, JOHN W. LEWIS