Superoxide dismutase (SOD) was present in 23 of 28 strains of the genus Bacteroides tested. Several clostridia, anaerobic cocci, and anaerobic, grampositive, nonsporing rods contained measureable SOD, but the frequency of SOD occurrence was much lower than in the bacteroides. These data indicate that there is a large variation in SOD levels between genera and among species within a genus of anaerobic bacteria. There was also no correlation between source of isolate, SOD levels, and presumed pathogenicity of the isolate.