A thermophilic, anaerobic, chemo-organotrophic sulfur-reducing bacterium, designated MV1075(T), was isolated from a deep-sea hydrothermal chimney sample collected on the Mid-Atlantic Ridge. Cells were rod-shaped with a sheath-like outer structure, motile with polar flagella and stained Gramnegative. They appeared singly, in pairs or in short chains. The temperature range for growth was 25-65 degreesC, with an optimum at 55 degreesC. Growth was observed from ph 5 to ph 9, and the optimum ph was around 7. The salinity range for growth was 15-70 g sea salt I-1 (corresponding to 10-45 g NaCl I-1), with an optimum at 30 g I-1 (20 g NaCl I-1). The isolate was able to grow on a broad spectrum of carbohydrates or complex proteinaceous substrates. Sulfur was not necessary for growth. Growth was inhibited by H-2, but, in presence of sulfur, this inhibition was removed and H2S was produced. The G+C content of the genomic DNA was 29 mol%. Phylogenetic analyses of the 16S rRNA gene located the strain within the order Thermotogales, in the domain Bacteria. On the basis of 165 rDNA sequence comparisons, in combination with morphological and physiological characteristics, it is proposed that the isolate should be described as a novel species of a new genus, Marinitoga gen. nov., of which Marinitoga camini sp. nov, is the type species. The type strain is MV1075(T) (= CNCM 1-2413(T) = D5M 13578(T)).