Enrichments for anaerobic organotrophic hyperthermophiles were performed with hydrothermal chimney samples collected at the Guaymas Basin (27° 01′ N, 111° 24′ W). Positive enrichments were submitted to γ-irradiation at a dose of 30 kGy. One of the resistant strains, designated strain EJ3T, formed regular motile cocci. The new strain grew between 55 and 95 °C, with an optimum growth temperature of 88 °C. The optimal pH for growth was 6·0, and the optimum NaCl concentration for growth was around 20 g l−1. Strain EJ3T was an obligately anaerobic heterotroph that utilized yeast extract, tryptone and peptone. Elemental sulfur or cystine was required for growth and reduced to hydrogen sulfide. The G+C content of the genomic DNA was 51·3 mol%. As determined by 16S rRNA gene sequence analysis, the organism was most closely related to Thermococcus celer, Thermococcus guaymasensis, Thermococcus hydrothermalis, Thermococcus profundus and Thermococcus gorgonarius. However, no significant homology was observed between them by DNA–DNA hybridization. The novel organism also possessed phenotypic traits that differ from those of its closest phylogenetic relatives. Therefore, it is proposed that this isolate, which constitutes the most radioresistant hyperthermophilic archaeon known to date, should be described as the type strain of a novel species, Thermococcus gammatolerans sp. nov. The type strain is EJ3T (=DSM 15229T=JCM 11827T).