Nuclear division and microgametogony of Eimeria nieschulzi were studied by transmission electron microscopy. All nuclear divisions occurred in close proximity to the gamont surface with four centrioles situated between the nuclear envelope and the gamont plasmalemma. During early nuclear divisions, each nucleus had a rod or hourglass-shaped nucleolus, indicating that the nucleolus probably pinched in two during nuclear division. Nuclear divisions occurred by nearly centrally located intranuclear spindles. Two centrioles were associated with each centrocone. Spindle microtubules, which originated from each centrocone, either traversed to the other centrocone or terminated at kinetochores. Four to 6 chromosome-like structures were present in each dividing nucleus. The formation of 50 to 100 biflagellate microgametes occurred at the margin of the microgamont by a process similar to that described for other coccidian species. Microgametes were limited by a single membrane with a prominent glycocalyx, 13 nm thick.