Treponema denticola strains ATCC 35405 and ASLM were found to have moderately active oxygen metabolism and consumed some 0.46 mumol O2/h/mg cell protein in anaerobic growth medium or about ten times this amount in aerobic medium. There appeared to be no differences between the two strains in their oxidative metabolism. The spirochetes showed significant endogenous O2 utilization, which was stimulated only slightly by added glucose or arginine, moderately by glycine, but markedly by casamino acids or brain-heart infusion broth. O2 metabolism by intact cells was insensitive to cyanide and so did not appear to involve cyanide-sensitive cytochrome oxidases. Moreover, difference spectra of cell extracts and membranes did not reveal heme profiles. However, the spirochetes did have very active reduced nicotinamide adenine dinucleotide (NADH) oxidase(s) and also contained the protective enzymes NADH peroxidase and superoxide dismutase. Both the oxidase(s) and the peroxidase had rather broad substrate specificities. Either NADH or reduced nicotinamide adenine dinucleotide phosphate could serve as reductant, and the enzymes were active with a variety of oxidants. Enzyme activity in fresh cell extracts was only somewhat stimulated by added flavins, but after frozen storage, the activity became much more activated by flavin adenine nucleotide, and to a lesser extent, by flavin mononucleotide. The enzymes were insensitive to fluoride, which inhibits heme-based but not flavin-based oxidases at low pH values. Clearly, these anaerobic spirochetes have significant oxygen metabolism, even at the low levels of O2 measured in periodontal pockets and contain enzymes that offer at least moderate protection against damage by reactive oxygen species.