A novel hyperthermophilic, anaerobic archaeon, strain Bio-pl-0405IT2(T), was isolated from an hydrothermal chimney sample collected from the East Pacific Rise at 2700 m depth on the "Sarah Spring" area (7°25'24 S, 107°47'66 W). Cells were irregular, motile cocci (0.8 to 1.5 µm in diameter) and divided by constriction. Growth was observed at temperatures between 60°C and 95°C with an optimum at 80°C. The pH range for growth was between 4 and 8 with an optimum around 7. Strain Bio-pl-0405IT2(T) grew at salt concentrations ranging from 1% to 5% (w/v) NaCl with an optimum at 2%. The novel isolate grew by fermentation or sulphur respiration on a variety of organic compounds. It was a chemoorganoheterotrophic archaeon growing preferentially with yeast extract, peptone and tryptone as carbon and energy sources and sulphur and organic compounds as electron acceptor; it grew also on maltose and starch. Sulphur or L-cystine were required for growth and were reduced to hydrogen sulfide. The strain was resistant to 100 µg/ml rifampicin, chloramphenicol, vancomycin and kanamycin but was sensitive to tetracyclin. The G+C content of its genomic DNA was 53.6 mol%. Phylogenetic analysis of the almost complete 16S rRNA gene sequence (1450 bp) of strain Bio-pl-0405IT2(T) showed that the novel isolate belonged to the genus Thermococcus. The DNA-DNA hybridization values with the two closest relatives T. hydrothermalis and T. celer were below the threshold value of 70%. On the basis of the physiological and genotypic distinctness, we propose a novel species Thermococcus prieurii sp. nov. The type strain is Bio-pl-0405IT2(T) (=CSURP577(T) =JCM16307(T) = LMM3069(T)).