In the salt marshes on the island of Sylt (North Sea), >100 species of free-living Plathelminthes have been found above mean high tide level. Such a high diversity was unexpected in a harsh and unpredictable environment. In order to resolve this paradox several experiments were conducted. Shortterm physiological resistance is high for variations in salinity and temperature. Vertical movements are performed to escape zones of low oxygen content or decreased water content. Horizontal migrations do not, however, occur and the above adaptations are insufficient to survive the range of conditions in these salt marshes. In sediment cores the moisture conditions, the salinity, and the temperature were manipulated. Different numbers of species were obtained from these treatments because species proved to be capable of encystment when environmental conditions became harsh, and to hatch as soon as conditions improved. At altered salinities and temperatures, different species hatched and became active. Other species endured seasonal desiccation and anoxia by remaining in egg-capsules. High environmental variability in these salt marshes, in spatial mosaics as well as temporarily, allows many species to find periods for activity, and encystment helps them to overcome intermittent harshness.