Traditionally benthic Plathelminthes were thought to be restricted to life in the sediment. However, recent studies revealed that many species may actively enter the water column. Experiments and field distribution data of Plathelminthes from an intertidal sandflat of the northern Wadden Sea (North Sea) in summer 1987 and 1988 are used to evaluate the specific composition of active swimmers and nonswimmers. It is shown that life-style changes continuously-from entirely benthic to semiplanktonic. With respect to swimming activity, phytal species connect semiplanktonic and entirely planktonic species. Morphology and ecology change in parallel to life-style. Consequently, the active swimming potential of Plathelminthes should be taken into account in both descriptive and experimental studies.