A novel coccoid-shaped, hyperthermophilic, heterotrophic member of the archaea was isolated from a shallow marine hydrothermal system at Vulcano Island, Italy. The isolate grew between 56 and 90 degrees C with an optimum around 85 degrees C. The pH range for growth was 6.5 to 10.5, with an optimum around 9.0. Polysulfide and elemental sulfur were reduced to H2S. Sulfur stimulated the growth rate. The isolate fermented yeast extract, peptone, meat extract, tryptone, and casein. Isovalerate, isobutyrate, propionate, acetate, CO2, NH3, and H2S (in the presence of S degrees ) were detected as end products. Growth was not inhibited by H2. Based on DNA-DNA hybridization and 16S rRNA partial sequences, the new isolate represents a new species of Thermococcus, which we named Thermococcus alcaliphilus. The type strain is isolate AEDII12 (DSM 10322).