This study was conducted with a wild oyster (Ostrea edulis) population obtained from natural settlement of the "Ria de Ortigueira." Oysters were distributed in three experimental stocks, two of them being kept in rafts and the third on an intertidal bed. Parasitological analyses were performed from May, 1986, to Dec, 1987. Bonamia ostreae was detected throughout the study, with the highest infection rate in Dec, 1987 (60.6% in stock A) and Oct, 1987 (57.1% in stock B and 64.3% in stock C). Cumulative mortality reached 94.6%, 89.5%, and 96.0% in stocks A, B, and C respectively. Differences among values of these parameters, infection rates, and cumulative mortality, however, were not statistically significant.