The genes in Butyrivibrio fibrisolvens that encode the enzymes involved in butyrate production were sequenced. In a type I strain (ATCC 19171T), the genes coding for the enzymes that catalyze the conversion from acetyl-CoA to butyryl-CoA, thl (thiolase), crt (crotonase), hbd (β-hydroxybutyryl-CoA dehydrogenase), bcd (butyryl-CoA dehydrogenase), etfB (electron transfer flavoprotein [ETF]-β), and etfA (ETF-α), were found to be clustered and arranged in this order. A type II strain (ATCC 51255) had the same clustered genes with the same arrangement, except that crt was not present in the clustered genes. The deduced amino acid sequences of these enzymes did not greatly differ between the two strains, and even between B. fibrisolvens and clostridia. Amino acid identity appeared to be higher within the same type than between types I and II. The clustered genes were shown to be cotranscribed, and constitutively transcribed without being affected significantly by culture conditions.