Extracellular products (ECP) produced by Vibrio anguillarum strain M3 originally isolated from diseased flounder ( Paralichthys olivaceus ) were prepared. ECP of M3 showed gelatinase, casinase, amylase and haemolytic activity on agarose plates. High protease activity against azocasin was detected. Bacterium M3 showed highest growth and protease activity at 25℃. The protease present in ECP showed maximal activity at pH 8 and 55℃; was completely inactivated by application of 80℃ heat for 30 min; was completely inhibited by EDTA and HgCl 2, and was partially inhibited by PMSF, SDS, MnCl 2 and iodoacetic acid; but not inhibited by CaCl 2 and MgCl 2. The ECP was toxic to flounder fish at LD 50 values of 3.1 μg protein /g body weight. The addition of HgCl 2 and application of heat at 50℃ decreased the lethal toxicity of ECP. When heated at 100℃, ECP lethality to flounder was completely inhibited. After intramuscular injection of ECP into flounder, it showed evident histopathological changes including necrosis of muscle, extensive deposition of haemosiderin in the spleen, dilated blood vessels congested with numerous lymphocytes in the liver. These results showed that ECP protease was a lethal factor produced by the bacterium V. anguillarum M3. Extracellular products (ECP) produced by Vibrio anguillarum strain M3 originally isolated from diseased flounder (Paralichthys olivaceus) were prepared. ECP of M3 showed gelatinase, casinase, amylase and haemolytic activity on agarose plates. High protease activity against azocasin was detected. Most complete and protease activity at 25 ° C. The protease present in ECP showed maximal activity at pH 8 and 55 ° C; was completely inactivated by application of 80 ° C heat for 30 min; was completely inhibited by EDTA and HgCl 2, and was partially inhibited by PMSF, SDS, MnCl 2 and iodoacetic acid; but not inhibited by CaCl 2 and MgCl 2. The ECP was toxic to flounder fish at LD 50 values ​​of 3.1 μg protein / g body weight. The addition of HgCl 2 and application of heat At 50 ° C reduced the lethal toxicity of ECP. When heated at 100 ° C, ECP lethality to flounder was completely inhibited. After intramuscular injection of ECP into flounder, it demonstrated evident histopathological changes including necrosis of muscle, extensive deposition of haemosiderin in the spleen, dilated blood vessels congested with numerous lymphocytes in the liver. These results showed that ECP protease was a lethal factor produced by the bacterium V. anguillarum M3.