Gamonts of Eimeria contorta were observed in intestinal epithelial cells of rats on days 6 and 7 postinfection. Early gamonts of both sexes were bound by a single limiting membrane, underlain locally by additional membranes. Mature microgamonts were limited by a single membrane as well, whereas macrogametes had a three-membrane pellicle. Amylopectin was found in gamonts of both sexes. Microgametes developed while connected to the microgamont's surface by a reinforced stalk. Flagellar basal bodies contained microtubules arranged in triplets. Macrogamonts contained labyrinthine wall-forming bodies (type 2) within cisternae of the rough endoplasmic reticulum before homogeneous wall-forming bodies (type 1) were found in the cytoplasm. In both sexes, the host cell membrane of the parasitophorous vacuole showed numerous blebs and intravacuolar folds; the latter appeared to branch occasionally. Intravacuolar tubules (IT) were connected to the surface of macrogametes and also interconnected with each other. The IT may have functions other than nutrient uptake. A third type of intravacuolar structure found within macrogametes resembled coils of longitudinally striated tubules. In cross section, these coils consisted of a series of microtubules interconnected by membranous bridges. In advanced stages, the coils were arranged parallel to the macrogamete's surface, thus forming an "exterior layer." They may have a role in oocyst wall formation.