Clostridia are well-known obligatory anaerobic bacteria which cannot utilize oxygen, or otherwise die in oxygenated environments. Clostridium butyricum, the type species of the genus Clostridium, possesses the ability to consume oxygen in amounts proportional to the size of the inoculum. Oxygen consumption was observed when NADH and NADPH were added to the cell extract of this strain. NADH oxidase and NADPH oxidase activities were also detected in all of the tested strains of the genus Clostridium. C. butyricum ceased growing while consuming oxygen in the medium. After consumption of all the dissolved oxygen, C. butyricum resumed growth at a rate equivalent to its anaerobic growth rate, suggesting that no oxidative damage based on oxygen reduction occurs in vivo. As scavengers for active oxygen species, the activities of NADH/NADPH peroxidase and SOD were detected in C. butyricum. Furthermore, the activities of these enzymes are distributed widely in the genus Clostridium.