Abstract We studied the cellular reactions of the polychaete Nereis diversicolor against different natural coelomic parasites: a yeast, a coccidian, and a supposed orthonectid. The coelomocytes engaged in these reactions are type 1 and type 2 granulocytes. We observed recognition processes, accumulation and flattening of coelomocytes into concentric sheets around the parasites, and then formation of thick capsules which become brown bodies. The parasites are first paralyzed (in case of motile stages), then show degeneration symptoms of the cell organelles, and are finally killed and turn into necrosed nodules. Encysted stages of coccidians may remain alive in the capsules. Life of small coccidian sporozoites is possible inside the coelomocytes.