A thermophilic, anaerobic, heterotrophic bacterium, designated 2PyrY55-1T, was isolated from the wall of an active hydrothermal white-smoker chimney in the Soria Moria vent field (71° N) at the Mohns Ridge in the Norwegian-Greenland Sea. Cells of the strain were Gram-negative, motile rods that possessed a polar flagellum and a sheath-like outer structure ('toga'). Growth was observed at 45-70 °C (optimum 65 °C), at pH 5.0-7.5 (optimum pH 5.5) and in 1.5-5.5 % (w/v) NaCl (optimum 2.5 %). The strain grew on pyruvate, complex proteinaceous substrates and various sugars. Cystine and elemental sulfur were used as electron acceptors, and sulfide was then produced. The G+C content of the genomic DNA was 27 mol% (Tm method). Cellular fatty acids included C16 : 0, C14 : 0, C16 : 1ω7c and/or iso-C15 : 0 2-OH, C16 : 1ω9c, C18 : 1ω9c, C18 : 0, C18 : 1ω7c and C12 : 0. Phylogenetic analyses of the 16S rRNA gene showed that the strain belonged to the genus Marinitoga in the family Petrotogaceae. Based on the phylogenetic and chemotaxonomic data, strain 2PyrY55-1T (=DSM 29778T=JCM 30566T) is the type strain of a novel species of the genus Marinitoga, for which the name Marinitoga arctica sp. nov. is proposed.