An autotrophic, hyperthermophilic methanogen (M7T) 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 H2 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 H2, 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-1T. 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 M7T (= DSM 12094T). During the course of this study, the 16S rDNA sequence analysis placed Methanococcus sp. strain AG86T (= DSM 4213T) 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 AG86T also constitutes a new species, Methanococcus fervens.