Crude membrane preparations from Fibrobacter succinogenes S85 were investigated and found to contain NADH dehydrogenase (NADH:decylubiquinone oxidoreductase) and NADH-linked fumarate reductase activities. Under aerobic conditions the maximum NADH dehydrogenase activity (252 nmoles/min/mg protein) was ten times greater than that of NADH-fumarate reductase (23 nmoles/min/mg protein). NADH-fumarate reductase was strongly inhibited by 2-heptyl-4-hydroxyquinoline-N-oxide (HOQNO), rotenone, HgC(l)2, and o-phenanthroline. Inhibition of the NADH dehydrogenase by the first three compounds, particularly rotenone, accounted for most of the effects on NADH-fumarate reductase. The alpha-band of a b-type cytochrome was resolved into two cytochromes, a cytochrome b(560) (oxidized by addition of HOQNO) and a cytochrome b(563) (oxidized by subsequent addition of fumarate).