An autotrophic, extremely thermophilic methanogen (MET) was isolated from a deep-sea hydrothermal chimney sample collected on the Mid-Atlantic Ridge at a depth of 3000 m. The heavily flagellated cells are motile and coccoid shaped. The new strain grows between 55 and 91 degrees C, with an optimum growth temperature at 85 degrees C. The optimum pH for growth is 6.5, and the optimum sea salt concentration for growth is around 25 g l(-1). The organism uses H-2 and CO2 as the only substrate for growth and methane production. Tungsten, selenium and yeast extract stimulate growth significantly. In the presence of CO2 and H-2, the organism reduces elemental sulphur to hydrogen sulphide. The G+C content of the genomic DNA is 33 mol%. As determined by 16S gene sequence analysis, this organism is closely related to Methanococcus jannaschii strain JAL-1(T). However, no significant homology was observed between them with DNA-DNA hybridization. It is proposed that this organism should be placed in a new species, Methanococcus infernus. The type strain is MET (= DSM 11812(T)).