A Muni employee found the man's body in the back of the bus, which had been parked in the Muni bus yard at 949 Presidio Ave. for several hours, at about 12:30 a.m. Saturday, police spokesman Sgt. Wilfred Williams said.
Williams said the body had no signs of trauma.
The medical examiner's office identified the man as 37-year-old Christopher Feasel, of San Francisco.
Williams said police are trying to determine whether Feasel was a passenger or entered the bus when it was parked in the yard.