A novel mesophilic bacterium, strain GO25(T), was isolated from a nest of hydrothermal vent polychaetes, Paralvinella sp., at the Iheya North field in the Mid-Okinawa Trough. Cells were motile short rods with a single polar flagellum. Growth was observed between 4 and 35 degrees C (optimum 30 degrees C; 13-16 h doubling time) and between pH 5.4 and 8.6 (optimum pH 6.1). The isolate was a facultatively anaerobic chemolithoautotroph capable of growth using molecular hydrogen, elemental sulfur or thiosulfate as the sole energy source, carbon dioxide as the sole carbon source, ammonium or nitrate as the sole nitrogen source and elemental sulfur, thiosulfate or yeast extract as the sole sulfur source. Strain GO25(T) represents the first deep-sea epsilonproteobacterium capable of growth by both hydrogen and sulfur oxidation. Nitrate or molecular oxygen (up to 10 % partial pressure) could serve as the sole electron acceptor to support growth. Metabolic products of nitrate reduction shifted in response to the electron donor provided. The G+C content of genomic DNA was 37.6 mol%. Phylogenetic analysis based on 16S rRNA gene sequences indicated that the novel isolate belonged to the genus Sulfurimonas and was most closely related to Sulfurimonas autotrophica OK10(T) (96.3 % sequence similarity). DNA-DNA hybridization demonstrated that the novel isolate could be differentiated genotypically from Sulfurimonas autotrophica OK10(T). On the basis of the physiological and molecular properties of the novel isolate, the name Sulfurimonas paralvinellae sp. nov. is proposed, with strain GO25(T) (=JCM 13212(T)=DSM 17229(T)) as the type strain. Thiomicrospira denitrificans DSM 1251(T) (=ATCC 33889(T)) is phylogenetically associated with Sulfurimonas autotrophica OK10(T) and Sulfurimonas paralvinellae GO25(T). Based on the phylogenetic relationship between Thiomicrospira denitrificans DSM 1251(T), Sulfurimonas autotrophica OK10(T) and Sulfurimonas paralvinellae GO25(T), we propose the reclassification of Thiomicrospira denitrificans as Sulfurimonas denitrificans comb. nov. (type strain DSM 1251(T)=ATCC 33889(T)). In addition, an emended description of the genus Sulfurimonas is proposed.