In this study the digenean parasites in the mud snail Hydrobia ulvae at two sites were compared for a period of one year. The sites were the north shore of Belfast Lough (Northern Ireland) and the Bend of Tarty on the Ythan Estuary (north-east Scotland). Samples of snails were collected monthly from which 13 species of digenean larvae were recorded at Belfast Lough. Two of these species were absent at the Bend of Tarty. Overall digenean prevalence was very similar at each site indicating that the respective bird communities were composed of ecologically equivalent species. Differences in the prevalence of individual digenean species are discussed in relation to the abundance of definitive and/or secondary intermediate hosts at each of the sites. Both sites displayed similar seasonal patterns of parasite prevalence with sharp increases to around 25% in the summer months. Snail shell heights were greater at Belfast Lough than at the Bend of Tarty and a positive correlation between snail shell height and infection by trematode parasites was recorded at both sites. Increased parasite prevalence in larger snails was attributed to factors such as age-related susceptibility and/or parasitic castration of the snails.