Snails of the species Thais (Nucella) lapillus (L) were collected from Scarborough South Bay, and Robin Hoods Bay, North Yorkshire. The presence of the rediae of Parorchia acanthus NICOLL (Digenea: PHILOPHTHALMIDAE) in T.lapillus individuals was previously associated with abnormal shell growth by Feare (1970a). His work has been extended to provide more conclusive evidence of parasitic gigantism in T.lapillus infested with P.acanthus. The energy increment and soft tissue mass increase associated with shell growth has been calculated for a sample of infested T.lapillus individuals. As reported by Cooley (1958) and Feare (1969) infestation with P.acanthus rediae progressively destroys the host gonad. The resultant reproductive saving was estimated for non-infested male and female T.lapillus from Robin Hoods Bay in 1981 and the energy values obtained were compared with estimates of the average energy loss from infested T. lapillus as a result of cercarial production and redial growth. The proportion of the whole body dry mass of infested T. lapillus individuals contributed by the redial population was generally similar to the gonadal proportion of non-infested femalest but did not follow the same seasonal cycle. The digestive gland of infested dogwhelks was proportionally reduced from that of non-infested females in August only. The growth of redial populations within the hosts through the summer is suggested as a possible cause of host gigantism. The relative advantages to the parasite of selecting female hosts are also discussed.