A novel mesophilic, hydrogen- and sulfur-oxidizing bacterium, designated strain NW8N, was collected from a sulfide chimney at the deep-sea hydrothermal vent on the Carlsberg Ridge of the Northwest Indian Ocean. The cells were Gram-stain-negative, motile, short rods with a single polar flagellum. The temperature, pH and salinity ranges for growth of strain NW8N were 4-40 °C (optimum, 33 °C), pH 4.5-7.5 (optimum, pH 5.5) and 340-680 mM NaCl (optimum, 510 mM). The isolate was an obligate chemolithoautotroph capable of growth using hydrogen, thiosulfate, sulfide or elemental sulphur as the sole energy source, carbon dioxide as the sole carbon source and molecular oxygen as the sole electron acceptor. The major cellular fatty acids of strain NW8N were summed feature 3 (C 7 and/or C 6), C and summed feature 8 (C 7 and/or C 6). The total size of its genome was 2 093 492 bp and the genomic DNA G+C content was 36.9 mol%. Phylogenetic analysis based on 16S rRNA gene sequences and core genes showed that the novel isolate belonged to the genus and was most closely related to GO25 (97.4 % sequence identity). The average nucleotide identity and DNA-DNAhybridization values between strain NW8N and GO25 was 77.8 and 21.1 %, respectively. Based on the phylogenetic, genomic and phenotypic data presented here, strain NW8N represents a novel species of the genus , for which the name sp. nov. is proposed, with the type strain NW8N (=MCCC 1A13988=KTCC 15780).