The autotrophic, sulfate-reducing bacterium, Desulfotomaculum orientis, grew in batch culture with molecular hydrogen (H-2) as an energy source, carbon dioxide (CO2) as a carbon source and sulfur dioxide (SO2) as the terminal electron acceptor. At high H-2 partial pressure, SO2 was stoichiometrically reduced to hydrogen sulfide (H2S). At low partial pressures of hydrogen (< 0.025 atm), SO2 was both oxidized to sulfate and reduced to hydrogen sulfide. These results indicated a new mode of sulfur metabolism for D. orientis.