A novel strictly anaerobic, thermophilic, sulfur-reducing bacterium, designated PH1209(T), was isolated from an East Pacific Rise hydrothermal vent (13 degrees N) sample and studied using a polyphasic taxonomic approach. Cells were Gram-negative, motile rods (approx. 1.60x0.40 mu m) with a single polar flagellum. Strain PH1209(T) grew at temperatures between 33 and 65 degrees C (optimum 60 degrees C), from pH 5.0 to 8.0 (optimum 6.0-6.5), and between 2 and 4% (w/v) NaCl (optimum 3%). Cells grew chemolithoautotrophically with H-2 as an energy source, So as an electron acceptor and CO2 as a carbon source. Strain PH1209(T) was also able to use peptone and yeast extract as carbon sources. The G+C content of the genomic DNA was 35 mol%. Phylogenetic: analyses based on 16S rRNA gene sequencing showed that strain PH1209(T) fell within the order Nautiliales, in the class Epsilonproteobacteria. Comparative 16S rRNA gene sequence analysis indicated that strain PH1209(T) belonged to the genus Nautilia and shared 97.2 and 98.7% 16S rRNA gene sequence identity, respectively, with the type strains of Nautilia lithotrophica and Nautilia profundicola. It is proposed, from the polyphasic evidence, that the strain represents a novel species, Nautilia abyssi sp. nov.; the type strain is PH1209(T) (=DSM 21157(T)=JCM 15390(T)).