Digestive gland protease pH optima and specific activities determined in Penaeus indicus with casein, azocasein, Azocoll, and Congo red fibrin as substrates were pH 7.7-9.2, 210-371 mu mol of tyrosine/mg of homogenate protein/min; pH 7.8, 36; pH 6.0-7.0, 7; and pH 8.9-9.2, 7A Delta0.001 U/mg of homogenate protein/min, respectively. Activity in the shrimp was stable during frozen storage but relatively labile and very low (1.043 azocasein units) in the Norwegian lobster, Nephrops norvegicus. The high activity in shrimp is significant in aquaculture and may be a source of proteolytic enzymes for industrial use. The rapid deterioration after landing may be a consequence of the high and stable activity. The low activity in the lobster may present a problem in culture and requires a more critical choice of feed as well as further investigation. 4-(2-Aminoethyl)-benzenesulfonyl fluoride hydrochloride was a very convenient, fast-acting, and effective inhibitor of shrimp trypsin and chymotrypsin but did not completely inhibit general protease activity in shrimp and had a negligible effect on the lobster. A significant component of that activity may be from nonserine proteases (such as the exoproteases carboxypeptidase A and B and the leucine aminopeptidases), whose proportion relative to the serine proteases may be greater in the lobster.