Although molecular ecology of macro parasites is still in its infancy, general patterns are beginning to emerge, e.g., that the most vagile host in a complex life cycle is the main determinant of population genetic structure in parasites. This insight stems from the observation that populations of parasites with only freshwater hosts are less structured than those with terrestrial or airborne hosts. Until now, the same has not yet been tested for marine systems, where in theory a fully marine life cycle might sustain high dispersal rates because of the absence of obvious physical barriers in the sea. Here,w e tested whether a marine trematode parasite that utilises migratory birds exhibited weaker population genetic structure than those whose life cycle utilises marine fish as the vagile host. Part of the mitochondrial cytochrome c oxidase 1 (COI) gene was sequenced from individual sporocysts from populations along the Atlantic coast of Europe and north Africa.