Microscpopically, the metacercariae of Clinostomum complanatum were encapsulated by a fibrous layer to form a cyst. The cyst wall was composed of connective tissue fibers, chiefly collagen fibers, of the fish in reaction to the infection. There were blood vessels penetrating into the cyst wall and forming a capillary network. These blood vessels might supply the nutritional requirements of the metacercaria and remove the wastes produced by the worms. The metacercaria of C. complanatum got a lot of nutrients from its host for growth and storage. The possible routes of the metacercaria of C. complanatum to get nutrients from its host may be through the digestive tract or by the direct absorption of the covering of the worm from the body fluid of the fish. The morphological feature suggested that the tegument of the metacercaria of C. complanatum possessed the tegumental absorption function. Generally, the metacercariae of C. complanatum lay quiescently in the cyst. No great harm was done to fish unless there were massive numbers. The greatest damage occurred when metacercariae had been activated and migrated out of the body wall of the fish. The process of the excystment and migration of the worms caused the congestion and hemorrhage followed by the serious tissue damage. After the excystment of the metacercaria, the fish tissues around the worm were dissolved. The result indicated that the worm might produce certain enzymes to facilitate the dissolution of fish tissues and cause the death of the fish due to the destruction of the fish body.