Several authors have directed their attention to the trematode parasites of species of Hydrobia, and their effects on the snail. MARKOWSKI (1936), M. ROTHSCHILD (1936, 1938, 1938a, 1941, 1941a) and SZIDAT (1943) concerned themselves with either Hydrobia stagnorum (Gmelin) [= H. stagnalis (Baster); H. ventrosa (Montagu)], or with H. ulvae (Pennant) (= Peringia ulvae). REES (1932) included Hydrobia jenkinsi in her survey of trematode larvae in Welsh snails, but with negative results. The Hydrobiidae — and H. ulvae in particular — are interesting objects of study for the parasitologist, because of their sensitivity for the presence of parasites and for the environment in which they live. Large numbers of H. stagnorum were collected by the author in the area of the Hondsbossche Zeewering (Province of Noord-Holland) and a survey of the habitats and parasites of H. ulvae was carried out on one of the Wadden Islands (Terschelling). In addition, a sample of H. ulvae was obtained from another of the Islands, Texel. The results of these surveys in the Netherlands are described in this paper and compared with the results published by the workers mentioned at the head of this Introduction.