A novel anaerobic, Gram-positive, sporulating and strictly chemoorganoheterotrophic bacterium, designated strain DV1 184 T, was isolated from a deep-sea hydrothermal vent sample from the Mid-Atlantic Ridge. The cells were short, straight rods (4 mum long and 0.8 mum wide) and were motile with peritrichous flagella. They grew between 35 and 55 degreesC (optimum, 45 degreesC), between pH 5.0 and 8.0 (optimum, 6.0) and at 20-60 g sea salts l(-1) (optimum sea salts concentration, 30 g l(-1)). Strain DV1184(T) was able to ferment yeast extract, tryptone, peptone, glucose, sucrose, maltose and pyruvate. The main fermentation products from glucose were (in decreasing order) formate, acetate and ethanol. The genomic DNA G+C content was 29 mol%. Phylogenetic analysis of the 16S rRNA gene located the strain within cluster XI of the lineage that encompasses the genus Clostridium and related genera in the bacterial domain. On the basis of 16S rDNA sequence comparison and physiological and biochemical characteristics, it is proposed that the isolate should be described as a novel species, Tepidibacter formicigenes sp. nov. The type strain is DV1184(T) (= CIP 107893(T) = DSM 15518(T)).