The redox enzymes in Propionibacterium arabinosum were examined. Succinate dehydrogenase was found mainly in the particulate fraction (membrane fragments), whereas NADH, α-glycerophosphate and D-lactate dehydrogenases [EC 1.6.99.3, 1.1.99.5 and 1.1.2.4] were distributed in both the particulate and soluble fractions. Comparison of the Km values and pH optima indicated that both cellular fractions contained the same species of α-glycerophosphate and lactate dehydrogenases. The particulate fraction also contained a b-type cytochrome, flavoproteins, non-heme iron, and phospholipidis. These components, together with the membrane-bound dehydrogenases, constituted a particular type of electron transfer system which oxidized NADH, α-glycerophosphate and lactate actively. The oxygen uptake with these substrates was considerably inhibited by the addition of fumarate. The nature of the terminal oxidase functioning in the membrane fragments remained to be solved. Based on these observations, a scheme was presented for the electron-transfer system in the membrane fragments of P. arabinosum. This system is similar to the NADH-cytochrome b and succinate-cytochrome b segments of mammalian mitochondria. It is postulated that succinate dehydrogenases in this system acts as fumarate reductase and is responsible for the reduction step of propionic acid fermentation.