The ultrastructure of microgametogenesis of Eimeria bakuensis was studied in the intestinal epithelium of an experimentally infected sheep. During development the nuclei became situated peripherally in the microgamont. Nuclear chromatin condensed into peripheral, dense portions that became the nuclei of the microgametes. The young microgamont was surrounded by a very narrow parasitophorous vacuole which widened during development and contained a few intravacuolar folds. Fissures and invaginations developed in the microgamont at the time the flagella began to grow. Polysaccharide granules were not present in the early and intermediate microgamonts but increased in number during the later stages and remained in the residual cytoplasm of the mature stage.