A thermophilic, anaerobic, chemolithoautotrophic bacterium, designated strain TB-2(T), was isolated from the walls of an active deep-sea hydrothermal vent chimney on the Mid-Atlantic Ridge at 36 degrees 14' N 33 degrees 54' W. The cells were Gram-negative rods approximately 1.5 microm in length and 0.75 microm in width. Strain TB-2(T) grew between 45 and 70 degrees C (optimum 55 degrees C), 10 and 40 g NaCl l(-1) (optimum 30 g l(-1)) and pH 4.5 and 7.5 (optimum pH 5.5). Generation time under optimal conditions was 50 min. Growth occurred under chemolithoautotrophic conditions with H(2) as the energy source and CO(2) as the carbon source. Nitrate or sulfur was used as the electron acceptor, with resulting production of ammonium and hydrogen sulfide, respectively. Oxygen, thiosulfate, sulfite, selenate and arsenate were not used as electron acceptors. Growth was inhibited by the presence of acetate, lactate, formate and peptone. The G+C content of the genomic DNA was 25.6 mol%. Phylogenetic analysis of the 16S rRNA gene sequence indicated that this organism is closely related to Caminibacter hydrogeniphilus and Caminibacter profundus (95.9 and 96.3 % similarity, respectively). On the basis of phylogenetic, physiological and genetic considerations, it is proposed that the organism represents a novel species within the genus Caminibacter, Caminibacter mediatlanticus sp. nov. The type strain is TB-2(T) (=DSM 16658(T)=JCM 12641(T)).