Dr. Riley Fisher Thomas, 90, a professor in the Howard University medical school from 1933 until he retired in 1970, died of a heart attack April 17 at the Perry Point Veterans Hospital in Perry Point, Md.

Dr. Thomas, who lived in Washington, was born in Port Gibson, Miss., and earned a bachelor's degree from Alcorn A&M University in Mississippi in 1915. He graduated from the University of Chicago medical school in 1932 and later earned a master's degree in medicine there.

During World War I, he was a second lieutenant in the Army.

Dr. Thomas came to Washington in 1933 and joined the Howard medical school as an instructor in physiology. He later became a social professor of medicine. He retired in 1959, but continued to work in the department of medicine until his second retirement in 1970. While at Howard, he designed a course in physical diagnosis and helped organize the diabetic clinic at the old Freedmen's Hospital.

Dr. Thomas' many professional memberships included the American Medical Association, the D.C. Medical Society, the Diabetes Association of D.C., and the Medico-Chirurgical Society of D.C.

Survivors include his wife of 51 years, Charlotte, of Washington; a stepson, Dr. Edward W. Hawthorne of Clarksville, Md., and five grandchildren.