During growth of ethanol plus sulfate Desulfovibrio gigas and three other Desulfovibrio strains tested contained high NAD-dependent alcohol dehydrogenase activities and dye-linked aldehyde dehydrogenase activities. In lactate-grown cells these activities were lower or absent. In D. gigas an NADH dehydrogenase activity was found which was higher during growth on ethanol than during growth on lactate. The NADH dehydrogenase activity appeared to consist of at least three different soluble enzymes. The aldehyde dehydrogenase activity in D. gigas was highest with benzylviologen as an acceptor and was strongly stimulated by potassium ions. Coenzyme A or phosphate dependency could not be shown, indicating that acetyl-CoA or acetyl phosphate are not intermediates in the conversion of acetaldehyde to acetate. In the absence of sulfate D. gigas was able to convert ethanol to acetate by means of interspecies hydrogen transfer to a methanogen. This conversion, however, did not lead to growth of the Desulfovibrio.