ABSTRACT. – We gathered field data to study the impact of the coypu (Myocastor coypus), an alien invasive species on the vegetation of a small coastal wetland of central Italy, studying the local diet composition, richness and seasonal turnover around a yearly cycle in the study area where the dominant vegetation includes some habitat types protected by the European Habitat Directive, such as rushbeds. We subdivided plant species in ecological plant categories and verified the hypothesis that coypu prefer feed on aquatic and semiaquatic plants (mainly monocotyledons). In rushbeds we identified 76 available plant species (51 dicotyledons and 25 monocotyledons) with significant seasonal differences. A high turnover in plant availability occurs between winter, the poorest seasons in terms of species richness, and spring, the richest season, following the local abrupt change in water level and the restarting of vegetative period. Around a yearly cycle, we recorded marked differences in plant availability in the four ecological plant categories during winter. Coypu’s diet was composed by a set of 19 plant species (mainly composed by hygrophilous monocotyledons and terrestrial dicotyledons), corresponding to about 25 % of the available plant species. We did not observed significant differences among consumed plant categories. Other than monocotyledons, diet included synanthropic dicotyledons largely occurring in the study area and macrophytes associated to Juncetalia maritimi rushbed, a habitat type of European conservation concern (e.g., Carex spp., Juncus spp., Bolboschenous maritimus).