The influence of water eutrophication on the macrophytic vegetation of Lake Lugano (Lago di Lugano) is demonstrated using two complementary procedures: - A comparative study of the flora, floristic diversity and abundance of vegetation of the three main basins (Northern, Southern, and Ponte Tresa) differing in their geographical and limnological (trophic) characteristics. Our findings are briefly compared with the observations made on other Swiss lakes. - A description of the qualitative and quantitative evolution of macrophytes based on the comparison of our results of 1980 and 1987 with those of Steiner in 1912. These three studies correspond to different trophic levels and provide trends to link the evolution of vegetation and flora to the phenomenon of eutrophication.