An extremely thermophilic, strictly anaerobic, facultatively chemolithoautotrophic bacterium designated strain DS(T) was isolated from Treshchinnyi Spring, one of the hottest springs of the Uzon Caldera (Kamchatka, Russia). Cells of the novel organism were Gram-negative rods, about 1.0-1.2 microm long and 0.5 microm wide. The temperature range for growth was 52-82 degrees C, with an optimum at 75 degrees C. Growth was observed at pH 6.8-7.4, and the optimum pH was 7.0-7.2. Strain DS(T) was able to grow lithoautotrophically with hydrogen in the presence of CO(2) as a carbon source and thiosulfate or elemental sulfur as an electron acceptor. It also grew well with ethanol, fumarate, succinate or malate in the presence of thiosulfate. Yeast extract was not required for growth and did not stimulate growth. The genomic DNA G+C content was 35.2 mol%. Phylogenetic analysis of the 16S rRNA gene sequence indicated that the novel organism was a member of the family Thermodesulfobacteriaceae. On the basis of phylogenetic and physiological considerations, it is proposed that strain DS(T) represents a new genus and species, Caldimicrobium rimae gen. nov., sp. nov. The type strain of Caldimicrobium rimae is DS(T) (=DSM 19393(T) =VKM B-2460(T)).