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The coccidian, Coelotropha durchoni, manages to develop in its host, the polycheate annelid, Nereis diversicolor, because of its ability to circumvent the host's internal defence system. First, it avoids phagocytes by penetrating other cells, principally eleocytes and muscular cells, where it undergoes a phase of intracellular development. After becoming extracellular, a thick coat protects it from being attacked by granulocytes. This coat then breaks and is shed from its surface to permit fertilization. Parasites that have lost their coats and remain unfertilized are surrounded with granulocytes and destroyed by encapsulation. A strict hormonal correlation exists between the biological cycles of the parasite and its host. Thus, the mature spores of the coccidian parasite are disseminated when the worm lays its gametes by rupture of teguments. C. durchoni and N. diversicolor have established a biological equilibrium that permits mutual survival of both partners and constitutes a simple model for the study of the host-parasite relationship.