A novel extreme thermophile was isolated from a water sample derived from a deep subsurface geothermal water pool at a depth of 1500 m in the Hacchoubaru geothermal plant in Oita Prefecture, Japan. The cells were found to be straight rods, each being motile by means of a polar flagellum. Growth was observed at temperatures between 60 and 85 degrees C (optimum 78 degrees C; 120 min doubling time) and between pH 5.5 and pH 9.0 (optimum 7.5). The isolate was a strictly aerobic heterotroph capable of utilizing a number of substrates such as yeast extract, peptone, tryptone, various carbohydrates, sugars, amino acids and organic acids. Elemental sulfur, thiosulfate, sulfide or cysteine-hydrochloride was required as an electron donor for growth. Hydrogen gas did not support growth. The G+C content of the genomic DNA was 44.7 mol%. Phylogenetic analysis based on 16S rDNA sequences and DNA-DNA hybridization analysis indicated that the isolate was closely related to members of the hydrogen-oxidizing, autotrophic and thermophilic genera Hydrogenobacter and Calderobacterium. However this isolate was differentiated from the previously described species of these genera on the basis of the physiological and molecular properties of the new isolate. The name Hydrogenobacter subterraneus sp. nov. is proposed; the type strain is HGP1T (= JCM 10560T = IFO 16485T).