The stock of the oyster Ostrea edulis in the Netherlands is located in an estuary on 2 sites, Yerseke Bank and Grevelingen. The Yerseke Bank became infected with the protozoan oyster pathogen Bonamia ostreae in 1980, as a result of importations of infected oysters from France. The oyster stock of the Grevelingen was free of bonamiasis until 1988. In 1989 maximum prevalence levels of 48% and maximum prevalences of dead oysters up to 80% were reached. A 5-year research programme has been started to monitor the spread of bonamiasis in the wild stock of the Grevelingen and to select the best management strategy for commercial production of O. edulis despite bonamiasis. From the first results it was concluded that bonamiasis was introduced into the Grevelingen by transport of infectious material, probably by ships. Further, it was concluded that the level of prevalence and development of bonamiasis is related in some way to (fishery) stress factors of the oysters and to environmental factors, but stocking density does not appear to affect the prevalence of bonamiasis.