Urastoma cyprinae is a turbellarian that infects the gills of numerous species of bivalves. In the current study we investigated the seasonal behavior and microhabitat preferences of this parasite in wild mussels (Mytilus galloprovincialis) from the Vigo estuary (northwest Spain). U. cyprinae exhibited a marked seasonal pattern, with the highest levels of infection occurring during summer and autumn, coinciding with the highest abundance of juvenile worms in the gills, and the lowest in winter. We observed a positive correlation between host size and parasite intensity, with the highest values of prevalence and intensity being detected in the largest mussels (>60 mm). The number of worms observed in the posterior region of the gills was significantly higher than that observed in the middle and anterior regions. Along the dorsoventral axis, worms were especially abundant in the ventral and middle regions. Implications of these findings for clarifying the life cycle of U. cyprinae in the Vigo estuary arc discussed.