The acetogens Sporomusa silvacetica, Moorella thermoacetica, Clostridium magnum, Acetobacterium woodii, and Thermoanaerobacter kivui (i) grew in both semisolid and liquid cultivation media containing O(2) and (ii) consumed small amounts of O(2). Low concentrations of O(2) caused a lag phase in growth but did not alter the ability of these acetogens to synthesize acetate via the acetyl coenzyme A pathway. Cell extracts of S. silvacetica, M. thermoacetica, and C. magnum contained peroxidase and NADH oxidase activities; catalase and superoxide dismutase activities were not detected.