A protease was isolated and purified from the supernatant of a culture of hyperthermophilic archaebacteria: Pyrococcus abyssi strain st 549. Purification consisted of three chromatographic steps. The enzyme purification yield was 4% and the purification factor 890. This protease is a seryl-protease hydrolyzing proteins and peptides with a preference for cleavage at the aromatic and hydrophobic residues in P1 and P'1 positions. Its activity is optimal at 95 degrees C and at pH 9. The electrophoretic mobility of the protease observed by zymogram suggests that it can adopt several oligomer forms. Three of them predominate displaying apparent molecular masses of 150, 105 and 60 kDa. Interdependence of the observed bands was revealed by changing the denaturation conditions of the samples (temperature, SDS concentration) before electrophoresis.