The primary aim of the present study was to conduct a 1-yr parasitological survey of a Mytilus galloprovincialis mussel population from the Aveiro Estuary, Portugal. In addition, we attempted to assess the host response to parasites by examining host histopathology, including an evaluation of hemocyte infiltration and parasite encapsulation. The parasites present (prevalence) included the following: 1 protozoan, Nematopsis sp. (70%); 1 turbellarian, Urastoma cyprinae, (39%); 2 trematodes, Diphtherostomum sp. (58%) and Prosorhynchus crucibulum (0.3%); 3 crustaceans, Bathylaophonte azorica (0.3%); and 2 species of Mytilicola sp. (3.5%), M. intestinalis and M. orientalis. Highest prevalences for the most abundant species occurred in summer and autumn. Histological sections of the mantle of infected mussels revealed a marked reduction in the reproductive tissue, indicating that P. crucibulum invades the mussel and promotes a severe damage to its reproductive capacity.