At the island of Sylt, North Sea, the plathelminth fauna of supratidal salt marshes was investigated. A total of 103 species is recorded, 75 of which were found throughout the year. These species are truely brackish. 78% of them perform univoltine life-cycles. Salinity, oxygen, and water availability are less stable in salt marshes than further seaward in the tidal zone. Many plathelminth species are able to encyst when conditions deteriorate. Thus they can withstand intermittend harshness. It is shown that every common plathelminth species prefers a distinct range of salinity. In species capable of encystment, every change in salinity causes a change in abundance. Regarding all plathelminth species, salinity affects the species composition. The plathelminth distribution pattern is determined by (1) salinity, (2) oxygen availability, and (3) temperature. Together with differences in ground relief, water content, sediment composition, vegetation density, and insolation these factors form a patchy environment. Sites of similar combination of factors are patchily distributed, and plythelminth distribution matches these mosaics. Thus, the distribution of species non-random in salt marshes. A modified index of 'mean crowding' (Lloyd 1967) is proposed to describe the distribution of species in patchy environments. From the distribution pattern, the combination of the factors preferred is extracted. Closely related species differ in the preferred ranges, and their niches overlap to a small extent only.