The presence of superoxide dismutase activity was demonstrated in the dialyzed cellfree extracts of lactobacilli which are considered to be obligatory anaerobes or microaerobes. Specific activities of Lactobacillus acidophilus A-28 YIT 0066, L. casei C-16 ATCC 7469, L. arabinosus ATCC 8014, L. casei S-1 YIT 0001 and L. plantarum 11-35 YIT 0101 were about 0.2-0.4 units/mg protein, which are much lower than those of Streptococcus faecalis and Escherichia coli. Activity was rather constant when L. acidophilus A-28 was grown aerobically in the MRS medium containing glucose, mannitol or sorbitol, though the cells grown with mannitol or sorbitol consumed more oxygen than the cells grown with glucose. Activity of the cells grown anaerobically may have constitutively definite activity sufficient for cell survival on exposure to aerobic conditions. The superoxide dismutase of lactobacilli is not a cupro-zinc enzyme, since it was not inhibited by 5 mM potassium cyanide.