A novel anaerobic, thermophilic and heterotrophic bacterium, designated strain DV114O(T), was isolated from a deep-sea hydrothermal vent sample from the Mid-Atlantic Ridge. The cells were non-motile straight rods, 1-8 mum long and 0-4 mum wide, surrounded by an outer sheath-like structure (toga). They grew at 45-80 degreesC (optimum 65 degreesC), pH 5-0-9-0 (optimum pH 6-0) and at sea salt concentrations of 20-60 gl(-1) (optimum 30 gl(-1)). Strain DV1140(T) was able to ferment yeast extract, peptone, brain heart infusion, gelatin, starch, galactose, arabinose, glucose, trehalose and cellobiose. The fermentation products identified on glucose in the presence of yeast extract and peptone were acetate, isovalerate and hydrogen. The G + C content of the genomic DNA was 33 mol%. Phylogenetic analysis of the 16S rRNA gene sequence (GenBank accession number AJ577471) located the strain within the genus Thermosipho in the bacterial domain. On the basis of 16S rRNA gene sequence comparisons, and physiological and biochemical characteristics, the isolate represents a novel species, for which the name Thermosipho atlanticus sp. nov. is proposed. The type strain is DV1140(T) (= CIP 108053(T) =DSM 15807(T)).