Nontraumatic gas gangrene is extremely rare. It usually occurs following ulceration or perforation of an abdominal or pelvic viscus in persons having impaired host immune defenses. A fatal case of nontraumatic Clostridium septicum septicemia and myonecrosis in an elderly diabetic man with a perforated colon carcinoma is presented. The case illustrates the occult manner in which such cases typically present and the fulminant course of this disease. The need for rapid diagnosis and the current recommended treatment of gas gangrene are discussed.