A survey was performed between June and September 2006 along the Portuguese coast to assess the prevalence of digenean parasitism in the netted whelk Nassarius reticulatus, a bioindicator of tributyltin (TBT) pollution. It was also intended to evaluate the effect of parasites on the reproduction of this gastropod species and their interference on the development of imposex (the superimposition of male characters onto prosobranch females) and male penis, based on field data. Five digenean species (Lepocreadium album, Gynaecotyla longiintestinata, Himasthla quissetensis, Diphtherostomum brusinae, and Cardiocephalus longicollis), plus 1 unidentified species, were found to infect N. reticulatus. Parasitism was spread along the Portuguese coast, but the higher values of prevalence were found in sheltered inshore areas where up to 67.4% of the animals were affected per sampling station. Parasitism has a castrating impact on the whelks and a reducing effect on male penis size, which causes serious disorders in the reproduction of N. reticulatus and may have an important impact in its population dynamics. However, no relationship between imposex severity and parasite infestation was found.