A hyperthermophilic archaeon, Thermococcus profundus DT5432, produced extracellular thermostable amylases. One of the amylases (amylase S) was purified to homogeneity by ammonium sulfate precipitation, DEAE-Toyopearl chromatography, and gel filtration on Superdex 200HR. The molecular weight of the enzyme was estimated to be 42,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The amylase exhibited maximal activity at pH 5.5 to 6.0 and was stable in the range of pH 5.9 to 9.8. The optimum temperature for the activity was 80(deg)C. Half-life of the enzyme was 3 h at 80(deg)C and 15 min at 90(deg)C. Thermostability of the enzyme was enhanced in the presence of 5 mM Ca(sup2+) or 0.5% soluble starch at temperatures above 80(deg)C. The enzyme activity was inhibited in the presence of 5 mM iodoacetic acid or 1 mM N-bromosuccinimide, suggesting that cysteine and tryptophan residues play an important role in the catalytic action. The amylase hydrolyzed soluble starch, amylose, amylopectin, and glycogen to produce maltose and maltotriose of (alpha)-configuration as the main products. Smaller amounts of larger maltooligosaccharides were also produced with a trace amount of glucose. Pullulan; (alpha)-, (beta)-, and (gamma)-cyclodextrins; maltose; and maltotriose were not hydrolyzed.