A survey is given of the parasites found in two species of Hydrobia, H. stagnorum from the Hondsbossche Zeewering in Noord- Holland and H. ulvae from the island of Terschelling. In H. stagnorum, Cercaria lebouri Stunkard, 1932, was found together with cysts of the same, and Cercaria markowskii n. sp. Both species are described. In H. ulvae, C. lebouri Stunkard, 1932, was re-discovered and C. angularis n. sp. described. An analysis is given of a simple survey of eight „habitats” on Terschelling where H. ulvae and its parasites were examined and the complexity of the ecology involved is emphasised. From this survey, the following tentative conclusions can be drawn: 1. Accessibility is probably the most important factor which determines whether the final hosts are infected from a population of intermediate hosts, and whether this population receives a further infection from the definitive hosts. 2. The behaviour of host, intermediate host and parasite is such that in a given environment their habits must sufficiently overlap to allow any life-cycles to be completed. Further, larval parasites have more requirements than either final or intermediate host, since both of these can live without the parasite (see „habitat” 7 for an example of this). 3. It does not seem likely, from the results obtained for the island of Terschelling, that parasites form a „limiting factor” for the populations of Hydrobia ulvae.