A moderately thermophilic, anaerobic, endospore-forming bacterium (strain SC 562T) was isolated from a hydrothermal vent chimney located at 13 degrees N on the East-Pacific Rise at a depth of 2650 m. Cells of strain SC 562T were straight to slightly curved rods, which were 0.7-0.9 microm in diameter and 3.5-6.0 microm in length with peritrichous flagella. Strain SC 562T formed round, refractile endospores in terminally swollen sporangia. The temperature range for growth was 33-60 degrees C, with an optimum at 50 degrees C. The pH range for growth was 4.8-8.5, with an optimum at pH 6.5-6.8. Growth of strain SC 562T was observed at NaCl concentrations ranging from 1.5 to 6% (w/v). The substrates utilized by strain SC 562T included casein, peptone, albumin, yeast extract, beef extract, alanine plus proline and starch. Glucose, maltose, pyruvate, valine and arginine each slightly stimulated growth in the presence of yeast extract. The products of glucose fermentation were ethanol, acetate, H2 and CO2. Strain SC 562T reduced elemental sulfur to hydrogen sulfide. The G + C content of the DNA of strain SC 562T was 24 mol%. 16S rDNA sequence analysis revealed that the isolated organism belonged to cluster XI of the Clostridium subphylum. On the basis of its physiological properties and phylogenetic analyses, it is proposed that strain SC 562T represents the sole species of a novel genus, Tepidibacter; the name Tepidibacter thalassicus is proposed for strain SC 562T (=DSM 15285T=UNIQEM 215T).