A thermophilic, anaerobic, strictly autotrophic, sulphur-reducing bacterium, designated BSA(T) (T = type strain), was isolated from a deep-sea hydrothermal chimney sample collected at the mid-Atlantic ridge. Gram-negative cells occurred singly or in pairs as small highly motile rods. Spores were not observed. The temperature range for growth was 40 to 75 degrees C, with an optimum at 70 degrees C, The ph range for growth at 70 degrees C was from 4.4 to 7.5, with an optimum around 6.0. The sea salt concentration range for growth was 15-70 g l(-1) with an optimum at 35 g l(-1). Elemental sulphur, thiosulphate and sulphite were reduced to hydrogen sulphide. Sulphate and cystine were not reduced. The G+C content of the genomic DMA was 35 mol%. Phylogenetic analyses of the 16S rRNA gene indicated that the strain was a member of the domain Bacteria and formed a branch that was almost equidistant from members of the orders Aquificales and Thermotogales, The new organism possesses phenotypic and phylogenetic traits that do not allow its classification as a member of any previously described genus; therefore, it is proposed that this isolate should be described as a member of a novel species of a new genus, Desulfurobacterium gen. nov., of which Desulfurobacterium thermolithotrophum sp. nov, is the type species. The type strain is BSA(T) (= DSM 11699(T)).