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 Arcachon from May 1989 to February 1991. One-year-old spat, free from disease 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. The influence of different rearing procedures on Ostrea edulis survival and development in four commercial batches of oysters, free of disease, was also studied.