Faunal composition and zonation of free-living Plathelminthes (Turbellaria) in salt marshes of the North Sea show a striking correspondence with the species distribution in the Baltic Sea. In both areas, salinity seems to be the key factor determining the distributional ranges. In the salt marches of the North Sea, salinity correlates with the small-scale species patterns, and the abundant species are shown to have preferred ranges of salinity. In the Baltic Sea, these species occupy the same ranges. It is proposed, that the plathelminth fauna of the Baltic Sea originates from the supralittoral zone of the North Sea which is inhabited by species confined to brackish water conditions. This hypothesis is corroborated by the high faunal similarity and the lack of Baltic endemics. While the macrofauna of the Baltic Sea is dominated by euryhaline marine species, it is suggested that the meiofauna is mainly composed of true brackish-water species.