Active, membrane-bound NADH and succinate oxidase activities with a temperature optimum of 75 ℃ were demonstrated in an extremely thermophilic bacterium. These were relatively stable in cell-free extracts and respiratory particles at 75 ℃, but at 90 ℃ the half-lives of these oxidase systems were about 15 min in respiratory particles and 80 min in cell-free extracts. The stability of the NADH oxidase in respiratory particles at 90 ℃ was enhanced by 2 M-(NH₄)₂SO₄, 50% (v/v) glycerol and by NADH. A number of other substrates were oxidized by the electron transport system. Respiratory particles contained cytochromes a-613, a-602, b-559, cytochrome o and at least one c-type cytochrome, c-555. The soluble fraction contained at least two c-type cytochromes, at least one of which was CO-reactive. The sensitivity of NADH and succinate oxidases to a range of inhibitors was determined.