A novel thermophilic, sulfur-reducing chemolithoautotroph, strain ABI70S6(T), was isolated from a deep-sea hydrothermal field at the Yonaguni Knoll IV, Southern Okinawa Trough. Cells of strain ABI70S6(T) were motile rods, 0.9-2.0 microm in length and 0.4-0.8 microm in width. Strain ABI70S6(T) was an obligately anaerobic chemolithotroph, exhibiting hydrogen oxidation coupled with sulfur reduction. Growth was observed at 55-78 degrees C (optimum, 70 degrees C), pH 5.0-7.5 (optimum, pH 5.5-6.0) and 0.5-4.5 % NaCl (optimum, 3.0 % NaCl). H(2) and elemental sulfur were utilized as electron donor and acceptor, respectively. The major fatty acids were C(16 : 0) (40.0 %) and C(20 : 1) (60.0 %). The G+C content of genomic DNA was 44.2 mol%. The physiological attributes of strain ABI70S6(T) are similar to those of species of genera within the family Desulfurobacteriaceae, most of which are thermophilic and chemolithoautotrophic sulfur reducers. However, 16S rRNA gene sequence similarities between the novel isolate and type strains of all species within the family Desulfurobacteriaceae were <87 %, which is close to the similarities found between the novel isolate and members of the family Thermodesulfobacteriaceae (<85 %). Based on physiological and phylogenetic features of the novel isolate, it is proposed that it represents a novel species in a novel genus, Thermosulfidibacter takaii gen. nov., sp. nov., within the phylum Aquificae. The type strain of T. takaii is ABI70S6(T) (=JCM 13301(T)=DSM 17441(T)).