A thermophilic, Gram-positive, endospore-forming, sulfate-reducing bacterial strain, designated RL80JIV(T), was isolated from a geothermally active underground mine in Japan. Cells were rod-shaped and motile. The temperature and pH ranges for growth were 61-80 degrees C (optimum at 69-72 degrees C) and pH 6.4-7.9 (optimum at pH 6.8-7.3), and the strain tolerated up to 0.5 % NaCl. Strain RL80JIV(T) utilized sulfate, sulfite, thiosulfate and elemental sulfur as electron acceptors. Electron donors utilized were H(2) in the presence of CO(2), and carboxylic acids. Fermentative growth occurred on lactate and pyruvate. The cell wall contained meso-diaminopimelic acid and the major respiratory isoprenoid quinone was menaquinone MK-7. Major whole-cell fatty acids were iso-C(15 : 0), iso-C(17 : 0) and C(16 : 0). Strain RL80JIV(T) was found to be affiliated with the thiosulfate-reducer Thermanaeromonas toyohensis DSM 14490(T) (90.9 % 16S rRNA gene sequence similarity) and with the sulfate-reducer Desulfotomaculum thermocisternum DSM 10259(T) (90.0 % similarity). Strain RL80JIV(T) is therefore considered to represent a novel species of a new genus, for which the name Desulfovirgula thermocuniculi gen. nov., sp. nov. is proposed. The type strain of Desulfovirgula thermocuniculi is RL80JIV(T) (=DSM 16036(T)=JCM 13928(T)).