An obligate piezophilic anaerobic hyperthermophilic archaeon (strain CH1T) was isolated from a hydrothermal vent site named "Ashadze" located on the Mid-Atlantic Ridge at 4100 m depth. Enrichment and isolation were carried out at 95°C under a hydrostatic pressure of 42 MPa. CH1T cells are highly motile irregular cocci with a diameter of about 1-1.5 µm. Growth was recorded within a temperature range of 80 to 108°C and a pressure range of 20 to 120 MPa. Temperature and pressure optima for growth were 98°C and 52 MPa respectively. No growth was observed under atmospheric pressures for temperatures ranging from 60 to 110°C. Growth was observed from pH 6 to 9.5 (optimum 7.5-8) and salt concentrations from 2.5 to 5.5 % NaCl (optimum 3.5%). Strain CH1T is strictly anaerobic, growing on complex proteinaceous substrates (yeast extract, peptone, casein) or sucrose, starch, chitin, pyruvate, acetate and glycerol, without any additional electron acceptors.The G+C content of the genomic DNA is 49.0 +/- 0.5 mol%. Analysis of the 16S rRNA gene revealed that strain CH1T belonged to the genus Pyrococcus. Based on similarity levels between ribosomal proteins and physiological properties it is proposed that strain CH1T represents a novel species, Pyrococcus yayanosii sp., nov. The type strain is CH1T (=JCM 16557). This strain is also available by request from the "Souchothèque de Bretagne" (catalogue LMBE) culture collection (collection no. 3310).