A hyperthermophilic archaebacterium was isolated from a deep-sea black smoker chimney (depth, 760 m) at the Minami-ensei Knoll (28 degrees 23'N, 127 degrees 38'E). The strain, designated DT1331, was a coccoid shaped bacterium about 0.5 to 1.0 microns in diameter. The cells were surrounded by a cell envelope. The temperature for growth was between 55 degrees C and 93 degrees C with an optimum 80 degrees C. The growth occurred from pH 4.5 to 8.5 and the optimum pH was 6.0. DT1331 required 1% to 5% NaCl for growth and cell lysis was observed below 1% NaCl concentration. The strain was an anaerobic chemoorganotroph requiring elemental sulfur obligately. Organic substrates used included tryptone, peptone, soytone, casein, gelatin, and yeast extract. Under the optimal conditions, DT1331 had a generation time of 50 min and could reach densities of about 1.5 x 10(8) cells/ml. DT1331 was resistant to ampicillin, chloramphenicol, erythromycin, kanamycin, streptomycin, and tetracycline, which was one of the common characteristics of archaebacteria. The G+C content of DT1331 was 52.3 mol%. Analysis of the 16S rRNA gene by restriction enzymes coincided with those of Thermococcus celer, indicating that this strain belonged to the genus Thermococcus.