The collagenolytic activity was assayed in the contents of the alimentary canal of rainbow trout, weighing about 200g, which had been given either eviscerated carp fingerlings or a commercial diet for rainbow trout. In both cases, practically no activity was detected in the contents of the stomach, whereas high activity was detected in the contents of the pyloric caeca and the intestine. When considered with the finding in a previous study that collagenase in the digestive system of fishes is possibly of pancreatic origin, this finding indicates that collagenase is secreted into the intestine at a point close to the pylorus. In order to ascertain further the participation of collagenase in the digestion of collagen by rainbow trout, the digestibility of the collagen in the freeze-dried carp meal was examined by an indirect method using chromic oxide. It was found that its apparent digestibility was about 45%, and that the ingested collagen was digested and absorbed in the course of passage through the pyloric caeca and the pre- and mid-intestine. These findings indicate that collagenase in the alimentary canal of rainbow trout participates in the digestion of the ingested collagen.