Trypsin appears to be the predominant protease in the midgut. Since the enzyme appears not to have been isolated, most authors have just tested for proteolytic activity over the pH range of 7 to 11 and reported their results as tryptic activity. The diffuse nature of the pancreas in most cases has limited many researchers to making relatively crude extracts from mixed tissues, hampering localization of the enzyme. Tryptic activity has been found in four stomachless species in Japan: Seriola, two basses and a puffer. Since these fish lack pepsin, some such kind of protease in the intestine would be the primary means of protein digestion. Tryptic activity was found in extracts of both the pancreas of perch and Tilapia and in intestinal extracts of Tilapia, all having a pH optimum of 8.0-8.2. Proteolytic activity has been identified in the pyloric caecae and intestine of rainbow trout. In grass carp, tryptic activity was stronger in the intestine than in the pancreas. In a mixture of pancreatic and pyloric caecae tissue from chinook salmon, casein was digested maximally at pH 9. Tryptic activity has also been demonstrated in extracts of liver of Several species, probably because in fish having a diffuse pancreas, pancreatic tissue extends into the liver, around the portal veins, and around the gall bladder. In several of the cases above, when extracts of pancreas were mixed with extracts of intestine, the tryptic activity increased ten-fold or more, suggesting the presence in fish of the enzyme enterokinase in the intestinal wall which activates in mammals the pancreatic trypsin as it reaches the intestine.