Peculiarities of pathogenesis of Gryllus bimaculatus fat body infected with Adelina grylli were investigated by means of histochemical methods and electron microscopy. The obtained results revealed the following. 1. Abrupt disappearance of glycogen from parasitized host cells, which takes place already at early stages of infection and is accompanied by accumulation of glycogen by parasites. 2. Gradual disappearance of fat drops from cytoplasm of infected cells, complete disappearance of fat drops being observed only if the tissue is filled with cyst stages and its integrity is actually distroyed. 3. The pathogenesis of insect coccidiose is closely connected with extraordinary hemocyte reaction of the host, apparently induced by parasites. Such a reaction results in the infiltration of the fat body with hemocytes and formation of melanized capsules around oocytes. In fact the activation of the hemocyte reaction is the transition from intracellular parasitism to the tissue one.