An extremely thermophilic archaeon, strain ST557T (T = type strain), was isolated from a deep-sea hydrothermal vent in the North Fiji Basin. This strain is a strictly anaerbic coccus whose cells are about 0.8 to 2 microns in diameter. The optimum temperature, pH and sea salt concentration for growth are 85 degrees C, 8.5, and 20 to 40 g/liter, respectively. Strain ST557T grows preferentially in the presence of elemental sulfur on proteinaceous substrates and on a mixture of 20 amino acids. It grows slowly on pyruvate and maltose. Growth is inhibited by rifampin. The DNA G + C content is 54 to 55 mol%. Sequencing of the 16S rRNA gene revealed that strain ST557T belongs to the genus Thermococcus. We propose that this organism should be placed in a new species, Thermococcus fumicolans.