Jabari, a Western Lowland Gorilla, was anesthetized and all indications were he was not experiencing any problems.
However officials say swelling in his throat blocked air from getting into his lungs, and he died.
Tests have determined that a bacterial infection on his tongue produced the swelling that led to his death. The source of the infection is unknown.
Jabari came to the Philadelphia Zoo from the St. Louis Zoo on July 13, 2004. He was 28 years old when he died.