A quantitative survey of the Plathelminthes living in soft mud in the intertidal zone of Konigshafen (isle of Sylt. North Sea) was conducted from November 1982 to June 1983 . Abundance was highly variable but high on the average (260 Individuals 10 . cm-2) . The number of species (49) and diversity was lower compared to adjacent sand flats . Most individuals (81 %) feed on diatoms. and average body size is small (90 % were < 1 mm in length) .
Pogaina suecica was abundant throughout the study period, while other species fluctuated strongly. Microstomum papilloszim showed a mass development in February and M. bioct~latumin May. Both species reproduce by paratomy. The rapidly reproducing Acoela Archaphanostoma agile and Philactinoposthia saliens were other abundant diatom-feeders. Abundant predators were Proxenetes quinquespinosus, P. quadrispinosus and Promesostoma gracilis. It is suggested that the high variability in the plathelminth assemblage of the mud flat is caused by fluctuations in the major food source (diatoms), and by the lack of refuges below the mud surface.