Entry mechanisms of the intrahemocytic parasite Bonamia ostreae (Ascetospora) into hemocytes of the sensitive (Ostrea edulis) and a resistant (Crassostrea gigas) oyster species have been analyzed. The study was based upon the development of a B. ostreae-hemocyte in vitro system. Ultrastructural features and Cytochalasin B sensitivity of the phenomenon demonstrate that B. ostreae enters into the two oyster species hemocytes by host-specified phagocytosis, although parasite contribution to entry into O. edulis hemocytes cannot be discarded. Possible receptors and the postphagocytic fate of the parasite are discussed. © 1992.