In this study we report the prevalence of Bonamia ostreae in populations of flat oyster, Ostrea edulis L., from different origins (natural settlement, hatchery, France, Italy, Greece, U.K.), growing on several areas of Galicia (NW Spain) (Atlantic coast), Murcia (SE Spain) and Delta of Ebro (NE Spain) (Mediterranean coast), during 1992. In Galicia B. ostreae was found in ten of the twenty sites analyzed. The parasites was not detected in the estuaries of Sada, Pontevedra and Vigo. Prevalences higher than 70% were found in the estuaries of Ribadeo, Viveiro and Ortigueira. In relation to the Mediterranean areas, B. ostreae was detected in one population from Murcia. This is the first time that B. ostreae is diagnosed in one ppulation of flat oyster from the Mediterranean coast of Spain.