A novel anaerobic, extremely thermophilic, facultatively chemolithoautotrophic bacterium designated strain SR(T) was isolated from a terrestrial hot spring in Kamchatka (Russia). The cells of the novel strain were spore-forming rods with a Gram-positive type of cell wall. The novel isolate grew at 60-82 degrees C (optimum 75 degrees C) and pH 6.0-7.5 (optimum 6.8). Molecular hydrogen and formate were used as electron donors. Thiosulfate, sulfate or elemental sulfur served as electron acceptors yielding hydrogen sulfide. No growth was observed on either substrate in the presence of nitrate as the electron acceptor. The G+C content of the DNA was 56.2 mol%. Phylogenetic analyses of the 16S rRNA gene showed that strain SR(T) was most closely related to Ammonifex degensii (96.4 % gene sequence similarity). However, the novel isolate possessed phenotypic characteristics that differed significantly from those of A. degensii, the only other recognized species of the genus Ammonifex. It is concluded that strain SR(T) (=DSM 19636(T)=VKM B-2461(T)) represents the type strain of a novel species of the genus Ammonifex, for which the name Ammonifex thiophilus sp. nov. is proposed. An emendation of the genus Ammonifex is proposed based on the phenotypic characteristics of the novel species.