A study of the growth and mortality of the European oyster Ostrea edulis, including the effects of Marteilia refringens and Bonamia ostreae, was carried out in three oceanic areas in the Bay of Areachon from May 1989 to February 1991. One-year-old spat, free from discase were used for the trials. Six months after beginning the trials, Marteilia refringens had severely infested the cultures. Although Bonamia ostreae was found during the first year, the rate of infestation was low. In the second year Bonamiasis increased and high mortality (>60%) occurred in Ostrea edulis populations. Although the growth of the oysters was good, their condition index was generally poor. Moreover, shell perforations by Polydora sp. were observed. In spring 1990, a low glycogen storage was noted which may have been related to the Marteilia refringens infestation although no relationship with the carbohydrate content and mortality was detected. The influence of different rearing procedures on Ostrea edulis survival and development in four commercial batches of oysters, free of discase, was also studied. The growth of Ostrea edulis in single culture was better than in cultures mixed with Crassostrea gigas. However, infestation by Marteilia refringens and Bonamia ostreae in the laid cultures was quite similar to that of the experimental cultures (bags). Due to the persistence of these two parasites, the culture of Ostrea edulis on a commercial scale is not advisable in the Bay of Arcachon.