An autotrophic, hyperthermophilic methanogen, strain SL43(T), was isolated from a deep-sea hydrothermal chimney sample collected on the Central Indian Ridge at a depth of 2420 M. The coccoid, surface-layer-carrying, Gram-negative-staining cells were heavily flagellated and exhibited a slight tumbling motility. The temperature range for growth at pH 6.5 was 50-86 degreesC, with optimum growth at 85 degreesC. The optimum pH for growth was 6-6 and the optimum NaCl concentration for growth was 30 g l(-1). The novel isolate used H-2 and CO2 as the only substrates for growth and produced methane. Selenium and yeast extract stimulated growth significantly. In the presence of CO2 and H-2, the organism reduced elemental sulfur to hydrogen sulfide. Growth was inhibited by chloramphenicol and rifampicin, but not by ampicillin, kanamycin, penicillin or streptomycin. The G + C content of the genomic DNA was 30.7 mol%. On the basis of 16S rRNA gene sequence analysis, this organism was most closely related to Methanocaldococcus infernus MET (3-2% distance). Its phylogenetic distinctiveness was confirmed by RFLP analysis of the 16S rDNA, a reliable tool for differentiating hyperthermophilic methanococci. On the basis of phylogenetic and physiological characteristics, it is proposed that strain SL43(T) (=DSM 15027(T) =JCM 11886(T)) be designated as the type strain of a novel species, Methanocaldococcus indicus sp. nov.