Clostridial myonecrosis or gas gangrene occurs most frequently in contaminated wounds following trauma or surgery. It is caused by a wide variety of Clostridium species, the most common being Clostridium perfringens. Spontaneous, non-traumatic clostridial myonecrosis is uncommon and is usually associated with gastrointestinal and haematological malignancy, diabetes mellitus and peripheral vascular disease. The case of a previously healthy 16-year-old boy with acute onset of gastrointestinal symptoms, who died of bacterial sepsis without apparent preceding trauma, is presented here. Clostridium fallax was identified as the most probable causative agent. As far as is known, this is the first report of fatal sepsis in humans due to C. fallax, which has been described only rarely as a cause of gas oedema in animals.