The present study was conducted in order to analyse the pathological situation, relating to the parasite Bonamia ostreae, in a number of stocks of flat oyster (Ostrea edulis L.) in different areas at the Galician coast (N.W. Spain) (estuaries of Ribadeo, San Ciprián, Ortigueira, Sada, Noya, Arosa and Vigo) during 1989 and 1990. The oyster stocks came from natural settlement on the Galician coast as well as importation from Italy, England or Norway. We found significant differences in the infection rate of B. ostreae in the different growing areas. Whereas, in some of them the parasite was not detected (estuaries of San Ciprián, Sada, Noya and Vigo), in other areas, the prevalence of Bonamia ostreae reached values close to 50% (estuaries of Ribadeo and Arosa) or higher than 70% (Ortigueira estuary).