The ultrastructural changes ofEimeria brunettiwhich occur during microgametogony in the small intestine of the domestic fowl are described. The development of the trophozoite into the microgametocyte was accompanied by cytoplasmic growth and nuclear division. The developing microgametocytes could be differentiated from developing schizonts because of ultrastructural differences in both cytoplasm and nuclei. The surface area of the microgametocyte was increased by deep invaginations of the limiting membrane. At this developmental stage, the nuclei were situated adjacent to this membrane and protrusions which developed, initiated microgamete formation. Two centrioles were positioned between each nucleus and the limiting membrane and were transformed into the basal bodies of the flagella. The flagella grew out from the basal bodies which then entered the protrusions. A mitochondrion and the dense chromatin‐containing portion of the nucleus also entered each protrusion. The microgametes matured while they were attached to the residual cytoplasm of the microgametocyte; they were finally budded off into the parasitophorous vacuole. The microgamete consists of an elongated nucleus which overlaps a mitochondrion towards the anterior of the cell. At this end, a dense plaque (perforatorium) is found together with the 2 basal bodies and their attached flagella. 5 microtubules were apparent and ran longitudinally from the basal body region. In cross section, they were arranged in a row of 4 with 1 diagonally opposite; only 2 of these microtubules extended to the posterior tip of the microgamet