The common whelk, Buccinum undatum L., was selected as a model host-parasite system in a seasonal study at an ‘accumulating’ sewage-sludge dump-site in the Firth of Clyde, Scotland, where marked spatial concentration gradients exist for a variety of trace contaminants in the sediments. A total of 2808 whelks was examined from seven sites over 18 months. The prevalence of larval digenean parasites increased significantly with distance from the dump site from approximately 2% on its periphery to 15.1% at a reference site 3 km from the centre. At a second reference site, 13 km from the dumping area, 19.7% of Buccinum were parasitized. The effects of site, season, sex and parasitism on growth and condition of Buccinum and exposure to sewage sludge on the age-prevalence relationship and host response to infection were examined. Age-frequency distributions of Buccinum were used to compare various sources of mortalities (natural, fishing, pollution and parasite-induced). Patterns in parasite prevalence recorded at the dump site were not correlated with any natural environmental or host-related factors that were examined. The gradient in parasitism in Buccinum is considered to result principally from the toxic effects of trace metals on the miracidium, reducing parasite transmission to the molluscan host. The Bucrinum-parasite system may therefore provide a sensitive and valuable biologically-based index of the dispersal of these contaminants around the sewage-sludge dump-site, and may have wider applications for marine environmental monitoring.