Nine thermophilic strains of aerobic, non-sporulating, heterotrophic bacteria were isolated after enrichment of chimney material sampled from a deep-sea hydrothermal field at a depth of 2634m on the East-Pacific Rise (1 °N). The bacteria stained Gram-negative. They were rod-shaped and measured approximately 0.5μm in width and 1.5-3.5μm in length. They grew at 55-80°C, pH 6-8 and 1-6 % NaCl. Optimal growth was observed at 70-75°C, pH7.0 and 1-3 % NaCl. The organisms were identified as members of the genus Rhodothermus, having a 16S rRNA gene similarity of 98.1 % with Rhodothermus marinus DSM 4252(T). The novel isolates differed morphologically, physiologically and chemotaxonomically from R. marinus, e.g. in lack of pigmentation, response to hydrostatic pressure, maximum growth temperature and DNA G+C content. DNA-DNA hybridization revealed a reassociation value of 37.2 % between strain PRI 2902(T) and R. marinus DSM 4252(T), which strongly suggested that they represent different species. Furthermore, AFLP fingerprinting separated the novel strains from R. marinus reference strains. It is therefore concluded that the strains described here should be classified as representatives of a novel species for which the name Rhodothermus profundi sp. nov. is proposed; the type strain is PRI 2902(T) (=DSM 22212(T) =JCM 15944(T)).