Proteases in grass shrimp (Penaeus monodon) digestive tract were extracted. Four fractions, A, B, C and D, demonstrated caseinolytic activity and were purified to electrophoretic homogeneity. A, C and D were trypsin-like, while B was a chymotrypsin-like protease. Optimal temperature for proteases A, B and C were 65°C, and that for D was 55°C for hydrolysis of casein. Optimal pH of proteases A and C was 8.0, and that of D was 7.0 for hydrolysis of p-toluenesulfonyl-L-arginine methyl ester. Optimal pH of protease B for hydrolysis of N-benzoyl-L-tyrosine ethyl ester was 8.0. Inactivation of 50 % enzyme activity in 5 min occurred at 67°C for protease B and 50°C for proteases A, C and D.