In the coastal waters of the Netherlands and in the German and Danish Wadden Sea, at present no PSP-producing toxic dinoflagellate blooms occur. This is in contrast with the situation in a number of countries from which Dutch traders import mussels (Mytilus edulis) and oysters (Ostrea edulis). Especially bulk imports of mussels entail a risk of the introduction of cysts of toxic dinoflagellate species. These can be present within the bivalves but, when dredged mussels are concerned, particularly in accompanying mud and sand. Such cysts can be released into the coastal waters when the imported bivalves are kept in open storage basins or relaid on plots on the sea bottom. To prevent introduction, a ban is in vigour on the immersion into Dutch coastal waters of any bivalve shellfish, originating from other areas than the Wadden Sea (Figure 1). To enable the shellfish industry to import bivalves in periods of low national production, the Netherlands Institute for Fisheries Research (RIVO-DLO) has designed recirculating storage systems in which mussels and oysters can be kept in quarantine. Up to 50 tonnes of mussels can be kept in good condition during at least three days and flat oysters can be stored for several weeks. To ensure elimination of any cells and cysts of dinoflagellates present, the effluent of the systems must pass filters which retain at least 99.9% of the particles larger than 20 mu m, the minimal diameter of potentially dangerous cysts. Eight of these quarantine systems for mussels and 18 for oysters were in operation by the end of 1992.