During the examination of Salmo gairdneriq Salmo, trutta and Thymallus thymallus from the Driffield section of the river Hull and its tributaries, Eastburn Beck and Driffield Beck,all fish were found to be infected with the cestode Qvathocephalus trunca. A prevalence of 2.2% was recorded for the procercoids of C.truncatus in the amphipod crustacean Gammarus pulex in the same habitat. Other helminth parasites recorded in both fish and gammarid hosts include Echinorhynchus truttae, Echinorhynchus salmonis, Neoechinorhynchus rutili, Cystidicola farionis, Cucullanus truttae and Crepidostomilm metoecuse. The life cycle of C. truncatus has been studied in the laboratory together with aspects of embryonic development, procercoid and adult morphology, establishment of infection and host-cestode interractions. Hexacanth embryos of C.truncatus were found to develop optimally in eggs cultured at between 15'0c and 20' 0C for about 25 days. Gammarus Pulex became infected only by swallowing egg capsules a containing hexcanth embryos fed to them on pieces of 6 lettuce leaves. The young developing embryo grows over a period of about 10 weeks to the infective procercold stage in the body cavity of the amphipod. In fish,the tapeworm forms an attachment in the distal end of a pyloric caecum 3 days after infection and matures in 8-10 days with production of eggs. By the 15th day the attachment to host tissue has become so firm that it is impossible to separate the worm from it. The tapeworm's hold on the caecal wall is probably achieved by the sucking effect of the funnel-shaped scolex supplemented by the spike-like microtriches of the inner scolex surface. The ultrastructure, of the soolex and body wall of strobila of both the procercoid and adult tapeworm have been described. Hydrolytic enzymes such as alkaline phosphatasep acid phosphatase and non-specific esterase have been localised in both the procercoid and adult tapeworm. Pathological effects of the tapeworm infection in fish are seen as swelling and proliferation of tissues of the caecal wall where the tapeworm forms an attachment. In long established infections erosion of the caecal wallq penetration by worms through the caecal wall into the body cavity and their attachment to the abdominal musculature of the fish host are common notable features.