Thiobscillus ferrooxidans is generally considered to be an autotrophic bacterium that can grow aerobically on soluble ferrous iron or sulfur compounds. We now report that the bacterium can grow on hydrogen by the dissimilatory reduction of soluble ferric iron under anaerobic conditions. A series of growth experiments was carried out under aerobic or anaerobic conditions with or without ferric iron using the strains. Strain IFO 14262 was shown to be capable of oxidizing hydrogen when grown under aerobic condition. This strain also grew on hydrogen with the reduction of ferric iron to ferrous iron under anaerobic conditions. The accumulation of ferrous iron was readily observed when the bacterium was grown by the dissimilatory reduction of ferric iron. Strain JCM 7811 did not grow on hydrogen under aerobic conditions, but could grow when incubated with hydrogen and soluble ferric iron under anaerobic conditions. Growth correlated with the amount of reduced ferric iron. It was found that four out of six different strains of T ferrooxidans, including strains IFO 14262 and JCM 7811, were able to grow anaerobically on hydrogen with ferric iron as the sole electron acceptor. The dissimilatory reduction of ferric iron may play an important role for growth under oxygen-limited conditions by T ferrooxidans.