There are indications from various studies that the formerly extensive fields of the hydroid colonies of Sertularia cupressina ('white weed') in the subtidal zone of the German part of the Wadden Sea have now completely disappeared. These fields were harvested year after year by the white weed fisheries for many decades, until the early 1970s. However, this exploitation is not likely to have caused the decrease, since it was given up for economic reasons at a time when the fields still existed. In search for other causes, laboratory experiments were carried out which revealed: (a) The motility of the larvae is of the order of only a few millimeters. The larvae seem to be dispersed in the Wadden Sea by tidal currents, (b) Settlement is possible on different kinds of hard substrate, even on grains of sand. This implies that settlement will be successful only on a substrate which is not displaced by the currents and which does not become intermittently covered by sediments such as shells and stones. The substrate is not actively selected by the larvae. The response to settle seems to be triggered by hydrodynamic conditions. (c) Most larvae settled within 12 hours of hatching, but most planulae started metamorphosis a few days after settling which indicated that the larvae had been in different stages of development at the time of release from the acrocyst. The results are discussed in the context of the hydrodynamic changes, as well as the changes in water quality and fishing effort of white-weed fisheries and shrimping in the Wadden Sea in the course of the century.