At a sandy beach site on the island of Sylt in the North Sea, the interstitial micro- and meiofauna has been studied by several investigators since the 1960s, and a total of 652 species living between the tide marks has been reported. Most of the effort was devoted to the free-living Plathelminthes ('Turbellaria'), which accounted for 32% of all species, followed by Nematoda (27%), Ciliata (11%, the smallest size-groups not included), Copepoda (10%), Gastrotricha (7%), and 9 other major taxa. The species ratio of macrofauna to interstitial fauna was about 1:25 for the entire beach. The investigated shore consists of a steep upper slope from high to mid tide line (13 m), and a gentle lower slope from mid to low tide line (100 m horizontal distance). While the macrofauna gradually increased in species number from the beach face (8 species) towards low tide line (23 species), this was not the case in the diverse interstitial fauna. It attained a broad maximum of species richness at a 10 m wide terrace just below the steep slope of the beach face. Here an optimal balance may exist between organic supply, oxygen and water retention. Three hundred and fifty species per meter interval of the transect were recorded. From there species richness declined gradually towards the low tide line (230 species) and abruptly at the steep slope of the beach face (170 species). Similarity analysis indicates a threshold in species composition at the bend between the gentle and the steep slope, which is just above mid tide level. For the marine interstitial fauna the mid shore provides a center of diversity.