Production of European flat oysters Ostrea edulis in the Netherlands has been hampered by the presence of the haplosporidian parasite Bonamia ostreae, which is now an enzootic species following its establishment after 1980. We analyzed histopathological data from annual shellfish disease monitoring from 1988 to 2006 to quantify prevalence of B. ostreae in flat oyster stocks of the marine Lake Grevelingen. In addition, we estimated prevalence of B. ostreae on a monthly basis with field surveys in 2003. The parasite was detected with PCR, using specific primers for B. ostreae. Prevalence of B. ostreae was analyzed relative to O. edulis density, biomass and a range of environmental parameters. B. ostreae was detected in flat oysters throughout the year with a higher prevalence in spring than in autumn, possibly due to termination of spawning and the onset of oyster growth in autumn. Although B. ostreae was detected in all oyster weight classes, prevalence was highest in the largest oysters in spring and declined disproportionately in autumn, possibly due to high mortality of large oysters before autumn, suggesting that prevalence depends on oyster age. Parasite prevalence was independent of oyster density and total biomass, but appeared to be higher after a warm autumn. Abundance of the flat oyster (infected or non-infected with B. ostreae) was negatively related to the temperature of the preceding period, suggesting that mortality in flat oysters increased at higher water temperatures. Furthermore, O. edulis appeared to be more susceptible to B. ostreae after years with lower food availability and lower salinities ( ; Production of European flat oysters Ostrea edulis in the Netherlands has been hampered by the presence of the haplosporidian parasite Bonamia ostreae, which is now an enzootic species following its establishment after 1980. We analyzed histopathological data from annual shellfish disease monitoring from 1988 to 2006 to quantify prevalence of B. ostreae in flat oyster stocks of the marine Lake Grevelingen. In addition, we estimated prevalence of B. ostreae on a monthly basis with field surveys in 2003. The parasite was detected with PCR, using specific primers for B. ostreae. Prevalence of B. ostreae was analyzed relative to O. edulis density, biomass and a range of environmental parameters. B. ostreae was detected in flat oysters throughout the year with a higher prevalence in spring than in autumn, possibly due to termination of spawning and the onset of oyster growth in autumn. Although B. ostreae was detected in all oyster weight classes, prevalence was highest in the largest oysters in spring and declined disproportionately in autumn, possibly due to high mortality of large oysters before autumn, suggesting that prevalence depends on oyster age. Parasite prevalence was independent of oyster density and total biomass, but appeared to be higher after a warm autumn. Abundance of the flat oyster (infected or non-infected with B. ostreae) was negatively related to the temperature of the preceding period, suggesting that mortality in flat oysters increased at higher water temperatures. Furthermore, O. edulis appeared to be more susceptible to B. ostreae after years with lower food availability and lower salinities (