A novel obligately anaerobic, extremely thermophilic, organotrophic bacterium, strain Rift-s3(T), was isolated from a deep-sea sample containing Riftia pachyptila sheath from Guaymas Basin, Gulf of California. Cells of the novel isolate were rods, 0.3-0.8 µm in width and 1.5-10 µm in length, surrounded by a sheath-like structure (toga). Strain Rift-s3(T) grew at temperatures ranging from 44 to 75 °C, at pH 5.5 to 8.0, and with NaCl concentrations of 3 to 60 g l(-1). Under optimum conditions (65 °C, pH 6.0, NaCl 25 g l(-1)), the doubling time was 30 min. The isolate was able to ferment mono-, oligo- and polysaccharides including cellulose, chitin, xylan and pectin, and proteins including β-keratins, casein and gelatin. Acetate, hydrogen and carbon dioxide were the main products of glucose fermentation. The G+C content of the DNA was 30 mol%. Phylogenetic analysis of 16S rRNA gene sequences showed the affiliation of strain Rift-s3(T) with the genus Thermosipho, with Thermosipho atlanticus Ob7(T) as the closest relative (96.5 % 16S rRNA gene sequence similarity). Based on the phylogenetic analysis and physiological properties of the novel isolate we propose a novel species of the genus Thermosipho, Thermosipho activus sp. nov., with Rift-s3(T) ( = DSM 26467(T) = VKM B-2803(T)) as the type strain.