At five localities in Purmerend, a rapidly growing town in the province of Noord-Holland, the Netherlands, the leaves of the White waterlily Nymphaea alba and the Yellow pondlily Nuphar lutea were checked for the presence of aquatic snails. Altogether 12 different species could be recorded. The speciescomposition differed very much front site to site. At one locality a mean density per leaf of 49.8 specimens of Ferrissia clessiniana, an invading species, could be registered.