Cell suspensions of H2/CO2-grown Sporomusa termitida catalyzed an H2-supported synthesis of acetate from CO2 at rates of about 1 μmol acetate x h-1 x mg protein-1. Cells pre-grown on methanol, mannitol, lactate, or glycine also displayed H2-supported acetogenesis from CO2, although at rates 5–85% that of H2/CO2-grown cells. With methanol-grown cell suspensions: the presence of methanol greatly stimulated the rate of H2-supported conversion of 14CO2 to 14C-acetate (which became labeled mainly in the COOH-group); and like-wise the presence of H2 stimulated the conversion of 14CH3OH+CO2 to 14C-acetate (which became labeled mainlyan the CH3-group). Analogous stimulatory effects were observed for cell suspensions pre-grown on methanol + CO2+H2. Furthermore, when H2 (+CO2) was included as a growth substrate with either methanol or lactate: both substrates were used simultaneously; there was no diauxie in the growth of cells or in acetate production; and the molar growth yield of S. termitida was close to that predicted from summation of the yields observed when grown with each substrate alone. These data indicated that S. termitida can grow by mixotrophy, i.e. by the simultaneous use of H2/CO2 and organic compounds for energy. Results are discussed in light of the ability of H2/CO2 acetogens to outprocess methanogens in H2 consumption in the hindgut fermentation of wood-feeding termites.