An extremely thermophilic archaeon, strain MA898, was isolated from a deep-sea hydrothermal vent on the Mid-Atlantic Ridge. This strain is a strictly anaerobic coccus of approximately 0.7-1.2 microm in diameter. Optimal temperature, pH, and NaCl concentration for growth are around 85 degrees C, pH 7, and 3%, respectively. Strain MA898 grows preferentially in the presence of elemental sulfur, polysulfur, cystine, or L-cysteine. The microorganism requires rich proteinaceous substrates. BHI-S medium supports rapid growth, with a final concentration of more than 1.2 x 10(9) cells ml(-1), but strain MA898 exhibits poor growth on 2216S medium (yeast/peptone) and poor growth on starch. Growth is inhibited by rifampicin and chloramphenicol at a concentration of 100 microg/ml. The DNA G+C content is 50 mol%. Sequencing of the 16S rRNA gene indicates that strain MA898 belongs to the Thermococcusgenus, and from DNA/DNA hybridization data it is proposed as a new species: Thermococcus atlanticus. The deposition numbers are CIP-107420T and DSM15226.