A collagenolytic activity capable of degrading native collagen under non-denaturing conditions was detected in the pyloric caeca of yellow-tail, Seriola quinqueradiata. The pyloric caecum enzyme was capable of reducing the specific viscosity of carp skin collagen at pH 7.5 and 20°C. Crude enzyme extracts from the pyloric caeca, which is a digestive organ, exhibited significant neutral protease activity. The effects of α-chymotrypsin and trypsin on the viscosity of collagen were rather small. However, the pyloric caecum enzyme produced marked viscosity reduction in collagen preparations. This indicates that a collageno-lytic enzyme, distinct from a-chymotrypsin and trypsin, may exist in the pyloric caecum extracts. The enzyme preparation was also capable of degrading native, reconstituted, carp skin collagen fibrils at pH 7.5 and 25°C, and insoluble bovine tendon collagen at pH 7.5 and 30°C. α-Chymotyrpsin and trypsin, each at a concentration equal to the amount of total protein in the enzyme preparation, had practically no effect on the release of bound hydroxyproline from collagen fibrils. The results corroborate the existence of a collagenolytic enzyme in the pyloric caeca of yellow-tail.