The ultrastructural changes occurring during macrogametogony in Eimeria brunetti were studied in tissue from the small intestine of infected young domestic fowls. As the macrogametocyte developed, an increase in the volume of the cytoplasm and the nucleus occurred. At this early stage of development the organism was limited by a single unit membrane which possessed a number of micropores. The organism was situated in a parasitophorous vacuole which contained intra-vacuolar folds and intra-vacuolar tubules. The wall forming bodies of type II (WFB II) started to appear before the wall forming bodies of Type I (WFB I). The WFB I developed in the cytoplasmic matric whereas the WFB II were formed within the cisternae of the rough endoplasmic reticulum apparently in association with Golgi bodies. The mature WFB I were homogeneous, limited by a unit membrane, and larger than WFB II which had no limiting membrane but remained surrounded by a membrane of the rough endoplasmic reticulum. Polysaccharide granules were formed in the cytoplasm between strands of the rough endoplasmic reticulum. During maturation the WFB I & II and the polysaccharide granules increased in size and number, and as this occurred a number of electron translucent vacuoles were ejected from the surface of the macrogametocyte. The mature macrogamete possessed a large nucleus with a nucleolus, a number of multi-membranous vacuoles, mitochondria, and canaliculi. The WFB I & II were located at the cell periphery and the polysaccharide granules further towards the interior of the organism. The organism was limited by a unit membrane but during macrogametogony a homogeneous layer had developed which extensively coated the macrog...