Protease activity from extracts of the stomach and hepatopancreas of the European lobster have been examined and compared. pH Activity profiles for hydrolysis of casein were similar for both extracts with optimal action at pH 2.3 and 5.8. The extracts of the hepatopancreas contained endoproteases—elastase (EC 3.4.21.36), trypsin (EC 3.4.21.4) and exoproteases—leucine aminopeptidase (EC 3.4.11.1) and carboxypeptidases a (EC 3.4.17.1) and carboxypeptidase b (EC 3.4.17.2). It was not possible to detect any chymotrypsin (EC 3.4.21.1) activity. The acidic protease activity (optimal at pH 2.3) did not hydrolyse N-acetyl phenylalanyl di-iodotyrosine (a pepsin-specific substrate) nor was the action at acid pH inhibited by the pepsin specific inhibitor pepstatin A. The physiological significance of this protease action at acid pH values is in doubt since it is not observed in other crustaceans nor was it detected in parallel experiments on whole gut extracts of the shrimp Penaeus monodon.