ABSTRACT.    Bonamiosis due to the intrahaemocytic protistan parasite Bonamia ostreae is a European endemic disease affecting flat oysters Ostrea edulis. This parasite is mainly intracellular, infecting the haemocytes, circulating cells especially involved in defence mechanisms of oyster. The parasite life cycle is not completely elucidated. However, direct infection between infected and healthy oysters is possible suggesting the absence of intermediate host in the achievement of the cycle. In this study, we investigated interactions between parasites and haemocytes by flow cytometry and light microcopy. In both approaches, analyses were performed after 2 hours of contact between haemocytes and parasites purified from highly infected flat oysters. Flow cytometry tests consisted in testing haemocyte activities including esterase activities, reactive oxygen species production and phagocytosis after contact with live parasites and with parasites inactivated by heating at 100 °C for 5 minutes. Two amounts of parasites per haemocyte (5/1 and 10/1) were tested and haemocytes alone were used as controls. Contact experiments were performed three times. Haemocytes in contact with parasites were also observed under light microscopy after cytocentrifugation. The results show a significant reduction of the esterase activities of the haemocytes in contact with parasites (active or inactive) as well as an inhibition of the free radicals production. The importance of this inhibition seems correlated with the quantity of parasites in contact with haemocytes. Lastly, no significant action of the parasite could be noted on phagocytosis after 2 hours of contact. Observations in microscopy support the results obtained by flow cytometry.