The biological parameters which generate seasonal fluctuations in the size of populations of Caryophyllaeus laticeps are examined in detail. A mathematical model is formulated to describe the dynamics of the adult parasite within the fish host Abramis brama, and the predictions of the model are compared with observed population data. The seasonality of parasite population size is shown to be caused by the combined effects of a temperature dependent mortality rate and flucate of larval parasites. The observed overdispersed distribution of adult parasite numbers within the host population is shown to be generated by variability in feeding habits between individual members of the fish population. The theoretical population model is used as a framework for the investigation of, among other factors, the comparative influences of the immigration and death rates on the dynamics of the adult parasite, the future behaviour of the system under altered environmental conditions and the importance of chance effects in the dynamics of individual parasite populations within a single host. The stability of the dynamics of the complete life-cycle of C. laticeps is discussed in relation to the inherent biological complexity of host/helminth parasite interactions.