Meronts usually have merozoites (sporozoites) in their cysts. A merozoite in sporozoon language is a motile stage. Mature Type I meronts with 8 merozoites first appearing at 12 h postinfection (PI) and measuring 5.0 x 4.9 microns. Mature Type II meronts with 4 merozoites and a large granular residuum first appeared 48 h PI and measured 5.1 x 5.1 microns. Type III meronts with 8 short merozoites and a large homogeneous residuum first appeared 72 h PI and measured 5.2 x 5.1 microns. Microgamonts (4.0 x 4.0 microns) produced about 16 microgametes that penetrated into macrogametes (4.7 x 4.7 micrometers). Macrogametes gave rise to 2 types of oocysts that sporulated within the host cells. Most were thick-walled oocysts (6.3 x 5.2 microns), the resistant forms that passed unaltered in the feces. Some were thin-walled oocysts whose wall (membrane) readily ruptured upon release from the host cell. Sporozoites from thin-walled oocysts were observed penetrating enterocytes in mucosal smears. The presence of thin-walled, autoinfective oocysts and the recycling of Type I meronts may explain why patients the development of some longlasting or relapsing intestinal infections.