Mesophilic, hydrogenotrophic, sulfate-reducing bacteria were isolated from a deep-sea hydrothermal chimney sample collected at 13 degrees N on the East-Pacific Rise at a depth of 2,600 m. Two strains (BL5 and H9) were found to be phylogenetically similar to Desulfovibrio profundus (similarity >99%), whereas two other strains (H1 and AM13T) were found to be phylogenetically distinct (similarity 96.4%) from Desulfovibrio zosterae, their closest relative. Strain AM13T was characterized further. It was a barophilic, Gram-negative, non-sporulating, motile, vibrio-shaped or sigmoid bacterium possessing desulfoviridin. It grew at temperatures ranging from 20 to 40 degrees C, with an optimum at 35 degrees C in the presence of 2.5% NaCl. The pH range for growth was 6.7-8.2 with an optimum around 7.8. Strain AM13T utilized H2/CO2, lactate, formate, ethanol, choline and glycerol as electron donors. Electron acceptors were sulfate, sulfite and thiosulfate, but not elemental sulfur or nitrate. The G + C content of DNA was 47 mol%. Strain AM13T (= DSM 14728T = CIP107303T) differed from D. zosterae not only phylogenetically, but also genomically (DNA-DNA reassociation value between the two bacteria was 23.8%) and phenotypically. This isolate is therefore proposed as the type strain of a novel species of the genus Desulfovibrio, Desulfovibrio hydrothermalis sp. nov.