An autotrophic, hyperthermophilic methanogen (M7(T)) was isolated from a deep-sea hydrothermal chimney sample collected on the East Pacific Rise at a depth of 2600 m. The coccoid-shaped cells are flagellated and exhibit a slight tumbling motility. The temperature range for growth at pH 6.5 was 49-89 degrees C, with optimum growth at 80 degrees C. The optimum ph for growth was 6.5, and the optimum NaCl concentration for growth was around 25 g l(-1). The new isolate used H-2 and CO2 as the only substrates for growth and methane production. Tungsten, selenium and yeast extract stimulated growth significantly. In the presence of CO2 and H-2, the organism reduced elemental sulphur to hydrogen sulphide. Growth was inhibited by chloramphenicol and rifampicin, but not by ampicillin, kanamycin, penicillin and streptomycin. The G+C content of the genomic DNA was 31 mol%. As determined by 16S rDNA gene sequence analysis, this organism was closely related to Methanococcus jannaschii strain JAL-1(T). However, despite the high percentage of similarity between their 16S rDNA sequences (97.1%), the DNA-DNA hybridization levels between these strains were less than 5%. On the basis of these observations and physiological traits, it is proposed that this organism should be placed in a new species, Methanococcus vulcanius. The type strain is M7(T)(= DSM 12094(T)), During the course of this study, the 16S rDNA sequence analysis placed Methanococcus sp. strain AG86(T) (= DSM 4213(T)) as a close relative of M, jannaschii strain JAL-1T. However, the weak level of DNA-DNA hybridization with this strain (< 10%) allowed the proposal that strain AG86(T) also constitutes a new species, Methanococcus fervens.