Scirpus americanus Pers., a rare species in Western Europe, seems to have a local optimum on the edge of ponds in salt marshes, where fresh water is running out of the dunes. Its ecology was studied in 1953 in a station situated on the “Green Beach” of the West-Frisian island of Terschelling, Netherlands. In the table are given 5 sample plot analyses of a transsect studied with the analytical method of Braun-Blanquet. They indicate that Scirpus americanus presents its local ecological optimum in an upper salt-marsh community of the Armerion maritimae, i.e. in a habitat with a salt content of the soil water not exceeding 9 g Cl per 1 except for some days per year. Specific important factors seem to be: (1) the neighbourhood of an open water surface; (2) a supply of fresh dune water as well as of tidal sea water; (3) the cutting of sods, which diminishes the competiting power of other species. The station was almost entirely destroyed in 1955 by measures taken in order to protect the coast against the attacks of the sea. A small part of the population remains, but its habitat has lost part of the specific qualities mentioned above. Its survival, therefore, is doubtful.