A novel anaerobic, moderately thermophilic, NaCl-requiring fermentative bacterium, strain OS1^T, was isolated from oil production water collected from Alaska, USA. Cells were Gram-negative, non-motile, non-spore-forming rods (1.7-2.7 x 0.4-0.5 μm). The G+C content of the genomic DNA of strain OS1T was 46.6 mol%. The optimum temperature, pH and NaCl concentration for growth of strain OS1T were 55°C, 7 and 10 g L^-1, respectively. The bacterium fermented D-fructose, D-glucose, D-maltose, D-mannose, α-ketoglutarate, L-glutamate, malonate, pyruvate, L-tartrate, L-asparagine, Casamino acids, L-cysteine, L-histidine, L-leucine, L-phenylalanine, L-serine, L-threonine, L-valine, inositol, inulin, tryptone and yeast extract. When grown on D-glucose, 3.86 mol hydrogen and 1.4 mol acetate were produced per mol of substrate. Thiosulfate, sulfur and Lcystine were reduced to sulfide, and crotonate was reduced to butyrate with glucose as the electron donor. 16S rRNA gene sequence analysis indicated that strain OS1^T was related to Anaerobaculum thermoterrenum (99.7% similarity), a member of the phylum Synergistetes. DNA-DNA hybridization for strain OS1T and A. thermoterrenum yielded a 68% relatedness value. Unlike A. thermoterrenum, strain OS1^T fermented malonate, D-maltose, tryptone, L-leucine and L-phenylalanine, but not citrate, fumarate, lactate, L-malate, glycerol, pectin or starch. The major cellular fatty acid of strain OS1^T was iso-C_{15 : 0} (91% of total). Strain OS1^T also had iso-C_{13 : 0} 3-OH (3%), which was absent from A. thermoterrenum, and iso-C13 : 0 (2%), which was absent from Anaerobaculum mobile. On the basis of these results, strain OS1^T represents a novel species of the genus Anaerobaculum, for which the name Anaerobaculum hydrogeniformans sp. nov. is proposed. The type strain is OS1 ^T (=DSM 22491^T =ATCC BAA-1850^T).