A description is given of the fine structure of the neogregarine Farinocystis tribolii in sporogony, and of parasite-host relations. After fusing of gametes a variable number of spherical zygotes are produced; there are usually eight, but their number may range from 3–10. The zygote is covered with a cytoplasmic membrane around which the wall of the oocyst develops. A large number of dark bodies and elongate mitochondria of the vesicular type are characteristic features of the cytoplasm of the zygote. The nucleus contains a promient osmiophilic nucleolus. In addition to developed zygotes, each gametocyst harbours a residual body with 2–7 nuclei, which autolyses later. During the period of spore formation from the zygotes we distinguished five sequential stages: a moderately ovoid stage, a navicular stage, an intensively staining stage, a uninucleate young spore (oocyst) and a mature spore with sporozoites. The fine structure of the moderately ovoid stage is similar to that of the zygote except that the wall of the former is thicker and both layers are undulate. The oocyst wall covering the navicular stages is deeply folded and thickened at the poles. Although the size of the dark included bodies did not change, they increased in number. The darkly staining stages are of irregular shape; they display a considerable affinity for iron haematoxylin, OsO4 and lead citrate. A staining layer deposited below the cytoplasmic membrane rises to the wall of the future spore. Typical of its cytoplasm was the presence of numerous canals and cisternae of endoplasmic reticulum. The wall of the uninucleate spore was thick, its cytoplasm condensed. There were plugs on both poles of the spore. Mature spores contained eight elongate sporozoites and a central cytoplasmic residual body without nuclei. There were no paranuclear bodies present in the cytoplasm of the sporozoites. At the start of infection, mitochondria of the host concentrate around schizonts and other immobile developmental stages of the parasite apparently as a kind of host response to the parasite. No other responses such as intensive phagocytosis or melanization were observed. After the consumption of the entire fat body of the host by the neogregarine only the nuclei of the fat cells remained in their original shape. The development of the host was stopped. If the infection is heavy the host dies.