From September 1994 to August 1995 we studied the diet of coypus, Myocastor coypus, in a freshwater habitat located in the Ticino regional park (NW Italy). Adult and young coypus were directly observed foraging in and near water. Aquatic macrophytes (81.8%) represented the staple food all year round. Common reeds (Phragmites australis) and Elodea spp. were the main components of the overall diet. A complementary seasonal consumption of submersed/floating-leaved macrophytes and emergent macrophytes was observed, the former ones reaching the highest value in summer (65.7%) when the latter ones were at their minimum (16.2%). Differences in diet composition of adult and young coypus were recorded in summer, when adults consumed more submersed/floating-leaved plants (mainly Callitriche stagnalis and Myriophyllum spicatum) and young coypus more terrestrial plants, predominantly black locust Robinia pseudoacacia leaves. Terrestrial vegetation was a usual integrative resource of the coypu diet all year round, particularly for young coypus in summer. Slight damage to the plant community was observed in small areas over-grazed by coypus. Nevertheless, 7 out of 12 threatened Italian hygrophilic species were eaten by coypus, suggesting that particularly sensitive aquatic plants could suffer from long–term foraging. Selective control of coypus in natural ecosystems of particular conservation concern is suggested, in order to limit damage to native plant communities of freshwater habitats.