Thermoanaerobacter (T.) brockii, T. ethanolicus, and T. thermohydrosulfuricus were tested for their capacities to oxidize H-2 in the presence of thiosulfate. T. brockii oxidized H-2 actively, while T. ethanolicus and T. thermohydrosulfuricus oxidized it poorly. At the end of the exponential growth, H-2 was oxidized by T. brockii in the presence of an energy source and thiosulfate. This oxidative process improved the growth of T. brockii. Thermoanaerobacter species could be divided into two groups with regard to their Hz metabolism in the presence of thiosulfate. Thiosulfate reduction by species of the genus Thermoanaerobacter is of significance in mineralizing organic matter in thermophilic environments.