An extracellular protease was purified from a deep-sea psychrophilic bacterium strain DY-A which was identified as a Pseudomonas species. The optimal growth and protease-producing temperatures of the strain were all 10°C, and the protease was secreted only at temperatures under 20°C. The enzyme was most active at 40°C and at pH 10.0. It was inhibited by phenylmethyl sulfonylfluoride and diisopropyl fluorophosphate, indicating that it is a serine protease. Chelators such as EDTA, EGTA, 1,10-phenanthroline and 2,2′-bipyridyl produced a decrease of activity. The enzyme was sensitive to denaturing agents such as SDS, urea, and guanidine HCl and resistant to thiol-containing reducing agents such as dithiotreitol. The enzyme was active towards N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilide and N-succinyl-Ala-Ala-Pro-Leu-p-nitroanilide. The native molecular mass of the enzyme determined by native PAGE and SDS-PAGE was 25 kDa.