Clostridium acetireducens is the first reported anaerobic bacterium that is dependent on acetate as an electron acceptor for growth on branched-chain amino acids and alanine. The fermentation pathway of leucine and its deamination product alpha-keroisocaproate were studied in this organism. Addition of Methanobacterium formicicum to pure cultures of C. acetireducens stimulated the degradation of alpha-ketoisocaproate but not the degradation of leucine, indicating that the electrons produced during the oxidative deamination of leucine were not transferred to hydrogen, This conclusion is supported by the observed low NAD(P)H ferredoxin reductase activity. Not only acetate but also crotonate proved to be an appropriate electron sink for the regeneration of NAD(P)(+) in this bacterium. Interestingly, C. acetireducens was shown to form polyhydroxybutyrate during growth on lecuine plus acetate.