A thermophillic, non-spore-forming, marine, sulfate-reducing bacterium, strain SL6(T), was isolated from deep-sea hydrothermal sulfides collected at Guaymas Basin. The Gram-negative-staining cells occurred singly or in pairs as small, highly motile rods. The temperature range for growth was 50-80degreesC with an optimum at 75degreesC. The pH range for growth at 70degreesC was 6.3-6.8, with an optimum at 6.5. The NaCl concentration range for growth was 5.55 g l(-1), with an optimum at 30 g l(-1). H-2 and CO2 were the only substrates for growth and sulfate reduction. However, growth was stimulated by several organic compounds. Sulfur, thiosulfate, sulfite, cystine, nitrate and fumarate were not used as electron acceptors. Pyruvate, lactate and malate did not support fermentative growth. Desulfoviridin was not detected. The G+C content of the genomic DNA was 28 mol%. On the basis of 16S rRNA sequence analysis, strain SL6(T) is related to members of the genus Thermodesulfobacterium. However, the novel organism possesses phenotypic and phylogenetic traits that differ from those of its closest relatives. Therefore, it is proposed that this isolate, which constitutes the first marine representative of this genus, should be described as the type strain of a novel species, Thermodesulfobacterium hydrogeniphilum sp. nov. The type strain is SL6(T) (= DSM 14290(T) = JCM 11239(T)). Because of the phenotypic characteristics of the novel species, it is also proposed that the description of the genus Thermodesulfobacterium requires emendation.