The taxonomy of the genus Plagiorchis Luhe, 1899 is reviewed with particular reference to the species P.elegans (Rud. 1802) and a list of Plagiorchia species previously recorded in Britain is given. The life cycle was initially established using Lymnaea stagnalis, Chironomus sp.larvae and LACA mice; morphology, intraspecific variation and host specificity at all levels of the life cycle have been examined and described. Besides L.stagnalis, L.palustris is susceptible to infection with P.elegans while L.peregra is refractory. Infections cause extensive damage to the digestive gland of L.stagnalis although they do not curtail its life span. Immature and mature specimens of L.stagnalis were infected; the former were castrated and the latter continued to reproduce. A single mother sporocyst of P.elegans produces approximately 650 daughter sporocyats from which develop several hundred thousand xiphidiocercariae. Upon release cercariae are negatively phototropic and negatively geotropic, with-light-and temperature affecting the rate of cercarial emmission. Cercariae encyst-in aquatic arthropods (Chironomus sp.,Asellus aquaticus and Gammarus pulex) and precociously in the snail host. Adult P.elegans range in size from 1.04 to 3.89mm by 0.34 to 0.96mm. Both mammals (mice, rats, gerbils and hamsters) and birds(chicks, ducklings and pigeons) are susceptible to infection. Egg counts were performed to follow the course of the infection in mice and rats. Primary infections of LACA mice are of short duration as a result of a host immune response. By means of surgical transplantation, the life span of P.elegans was increased from the expected 21 days to 63 days. Specimens recovered when the life cycle was completed using various combinations of intermediate and final hosts and those recovered from first and second challenge infections of LACA mice are compared statistically using canonical variate analysis. The only reliable anatomical criteria for distinguishing species of Plagiorchis are: 1. the relative sizes of the suckers and pharynx. 2. the presence or absence of a seminal receptacle, vas deferens and common genital atrium. 3. to a limited extent egg size and host specificity. According to these criteria many previously described species are considered to be synonymous with P.elegans and its geographical range is extensive.