Bonamia ostreae, an intrahemocytic protozoan parasite infecting the oyster Ostrea edulis, is responsible for an endemic disease affecting all intensive breeding areas of European countries. Epidemiological studies showed variable prevalences, 43% in Galicia (Spain), from 6% to 32% in France and 45% in the most important area of Ireland. The impact of oyster transfers, handlings, high densities have been implicated in the disease development but environmental stressors could be more involved than zootechnical methods. The effect of water temperature, salinity and tidal exposure were particulary examined to assess stress induced changes and determine physiological, biochemical and immunological state indices of oyster populations, which will contribute to progress in the establishment of prophylactic measures. With Bonamia purification protocol, experimental infections have permitted to study bonamiosis development kinetics, confirming a latent period with unknown location of the parasite within the animal tissues. Concerning hemocyte-parasite interactions studies, using in vitro hemocyte infections and chemiluminescence technique, it has been shown that Bonamia counteracts the oxidative defence system in both O. edulis and C. gigas. The question of specificity barrier and elimination of B. ostreae in Japanese oyster remains unresolved. Finally, experimental infections allowed to undertake a selection program for Bonamia resistant oysters. This could be the most effective strategy to ensure uninterupted production of flat oysters.