THE homology in primary structure of mammalian pancreatic serine proteases is well established1,2. Progress has now been made in the elucidation of the evolutionary history of this group of proteases. Neurath and his co-workers3 studied the amino-acid composition of trypsinogen isolated from the pancreas of the spiny Pacific dogfish and presented a preliminary examination of the primary structure of this enzyme. From these data a high degree of homology as compared with bovine trypsinogen can be inferred. It is now of interest what will be found beyond the gap that separates vertebrate and invertebrate animals. Invertebrate proteases are difficult to investigate because of the small amounts of material obtainable from these animals. In the crayfish Astacus leptodactylus we found an enzyme resembling mammalian trypsin in many respects4,5. It exhibits a distinct tryptic cleavage specificity when tried on the oxidized B-chain of insulin and is inhibited by all natural and synthetic trypsin inhibitors including TLCK. It is a serine protease and possesses the sequence –Asp–Ser–Gly– around the active centre serine residue6. In addition, extended immunological studies have been carried out on the crayfish trypsins of three species which revealed that they do occur as iso-enzymes varying in number and electrophoretic mobility7.