In 1982 and 1983, sheltered upper intertidal and supratidal habitats on Sylt island were quantitatively studied for platyhelminths. This included (1) monthy sampling of two accretion sites, one vegetated and the other bare, and two salt marsh sites, one grazed and the other ungrazed; (2) seasonal sampling of 4 transects across salt marsh areas and accretion zones; and (3) some 50 sites sampled once only. In the accretion zone, the main factors stucturing platyhelminth assemblages were sediment type (sand or mud), the frequency of flooding as determined by tidal level (twice a day, during spring tides, or during storm floods only), and the density of vegetation. In supratidal salt marshes these factors were overruled by salinity and sediment humidity.