The fine structure of the changes occurring during microgametogenesis and of the microgametes of Eimeria acervulina was studied in experimentally infected chickens. Early microgametocytes had relatively electron-transparent nuclei with few, scattered, dense masses. Micropores were seen at the surface of these microgametocytes. Intranuclear microtubules extended inwards from a nuclear protuberance (centrocone) oriented towards the cell membrane. One to several fissures appeared in the cytoplasm of the microgametocytes and the nuclei oriented themselves along either the outer membrane or a fissure. One or 2 centrioles were seen between the nuclei and the cell membrane at this stage. Later, flagella and their basal bodies were seen in this position. A wide, dense band appeared in the nucleus perpendicular to the surface nearest the flagella. Later, the pale portion of the nucleus was pinched off and remained within the residual body. Mature microgametes had an anterior perforatorium, 3 basal bodies, 3 flagella, 4 microtubules running adjacent to the anterior mitochondrion, and a dense, elongated nucleus mostly posterior to this, with a narrow portion extending anteriorly nearly as far as the anterior end of the mitochondrion.