Butyryl-CoA, crotonyl-CoA or 3-hydroxybutyryl-CoA all ultimately form an enzyme-acetoacetyl-CoA complex upon aerobic addition to butyryl-CoA dehydrogenase purified from Megasphaera elsdenii, implying the presence of crotonase activity. This behaviour remains even after treatment with 6M urea, which destroys the activity of the main crotonase fraction from M. elsdenii. Flavin-sensitised photoinactivation destroys residual crotonase and dehydrogenase activities in parallel. Butyryl-CoA dehydrogenase thus has intrinsic crotonase activity with a turnover rate (0.05 min-1) about 0.02% of the figure for dehydrogenase activity. Mechanistic implications are discussed.