In a previous paper, we examined the toxicity of the extracellular protease of A. salmonicida, Ar-4 (EFDL) on yamabe (Oncorhynchus masou f. ishikawai) and goldfish (Carassius auratus). From these results, we considered the protease secreted by this bacterium was a causative agent of furunculosis. In this paper, we observed enzymatic properties of the purified protease. The results obtained were summarized as follows: 1. The molecular weight of purified protease was estimated to be 71, 000 by sodium dodecyl sulfate polyacrylamide gel electrophoresis. 2. The purified protease showed maximal activity at pH 9.4 and 50°C. It was stable over the pH range 5.0 to 10.0 and was completely inactivated by temperature at 56°C. 3. The protease was presumably classified an alkaline serine protease and it showed chymotryptic properties since it was significantly inhibited by diisopropylfluorophosphate (DFP) and tosyl-phenylalanine-chloromethyl ketone (TPCK) and hydrolyzed N-benzoyl-L-tyrosine ethyl ester (BTEE).