A thermophilic, anaerobic, chemo-organotrophic bacterium, designated MV1087(T), was isolated from a deep-sea hydrothermal chimney sample collected from the Mid-Atlantic Ridge. The cells were straight, motile and stained Gram-negative. Growth was observed from 45 to 65 degreesC, with an optimum around 65 degreesC. No growth was observed at 40 or 70 degreesC. Growth was observed from pH 5.5 to 9.0 and the optimum pH was around 7. The salinity range for growth was 10-100 g sea salt l(-1) (corresponding to 6.5-65 g NaCl l(-1)) with an optimum at 30 g sea salt l(-1) (20 g NaCl l(-1)). Strain MV1087(T) was heterotrophic, able to ferment proteinaceous substrates, such as brain/heart infusion and gluten, and carbohydrates, such as glucose, xylan and starch. The DNA G+C content was 27 mol%. Phylogenetic analyses using 16S rDNA sequences indicated that strain MV1087(T) belonged to cluster XII of the Clostridium subphylum. Due to its phenotypic and genotypic characteristics, isolate MV1087(T) is proposed as a novel species of a new genus, Caloranaerobacter azorensis gen. nov., sp. nov. The type strain is MV1087(T) (= CNCM I-2543(T) = DSM 13643(T)).