A thermophilic, marine, anaerobic, chemolithoautotrophic, sulfate-reducing bacterium, strain CIR29812(T), was isolated from a deep-sea hydrothermal vent site at the Kairei vent field on the Central Indian Ridge. Cells were Gram-negative motile rods that did not form spores. The temperature range for growth was 55-80 degreesC, with an optimum at 70 degreesC. The NaCl concentration range for growth was 10-35 g l(-1), with an optimum at 25 g l(-1). The pH range for growth was 6-6(.)7, with an optimum at approximately pH 6(.)25. H-2 and CO2 were the only electron donor and carbon source found to support growth of the strain. However, several organic compounds were stimulatory for growth. Sulfate was used as electron acceptor, whereas elemental sulfur, thiosulfate, sulfite, cystine, nitrate and fumarate were not. No fermentative growth was observed with malate, pyruvate or lactate. The phenotypic characteristics of strain CIR29812(T) were similar to those of Therm odesulfobacterium hydrogeniphilum, a recently described thermophilic, chemolithoautotrophic sulfate-reducer. However, phylogenetic; analyses of the 16S rRNA gene sequences showed that the new isolate was distantly related to members of the family Thermodesuffobacteriaceae (similarity values of less than 90%). The chemotaxonomic data (fatty acids and polar lipids composition) also indicated that strain CIR29812(T) Could be distinguished from Thermodesulfobacterium commune, the type species of the type genus of the family Thermodesulfobacteriaceae. Finally, the G + C content of the genomic DNA of strain CIR29812(T) (46(.)0 mol%) was not in the range of values obtained for members of this family. On the basis of phenotypic, chemotaxonomic and genomic features, it is proposed that strain CIR29812(T) represents a novel species of a new genus, Thermodesulfatator, of which Thermodesulfatator indicus is the type species. The type strain is CIR29812(T) (= DSM 15286(T) = JCM 11887(T)).