Based on parasite purification and hemocyte primary culture, an in vitro system was established for studying early interactions between Bonamia ostreae and the hemocytes of sensitive Ostrea edulis and resistant Crassostrea gigas. Infections were observed whichever the hemocytic types and the oyster species. The advantages and disadvantages of the system are discussed with regard to the study of recognition, entry, and survival mechanisms of the parasite. © 1992.