The ultrastructural changes occurring during wall formation in the oocysts of E. brunetti were studied in tissue from the small intestine of young domestic fowls. The process of oocyst wall formation was initiated by a separation of the limiting membranes of the cytoplasmic mass of the macrogamete, to form a loose veil, consisting of 2-3 membranes, around the organism. Concomitant with the appearance of the veil, changes were observed in the Wall Forming Bodies of Type I (WFB I) and Type II (WFB II) which were present in the cytoplasm of the macrogamete. The outer layer of the oocyst wall seemed to be formed by a combination of membranes released from the surface of the organism and the contents of the WFB I. A 700 nm thick layer was thereby formed and, at this stage, the WFB I had disappeared from the cytoplasmic mass. The outer layer further condensed into a structure 250-350 nm in thickness in which 3 zones of different densities could be distinguished. The formation of the inner layer of the oocyst wall appeared to be associated with material from the WFB II and these disappeared from the cytoplasmic mass as the inner layer became evident. This inner layer was homogeneous in appearance and 80 nm thick. Comparison of the ultrastructure of the walls of oocysts obtained from chicken faeces with that of oocysts observed within the intestine showed only minor differences. The loose veil was not present in oocysts isolated from faeces and some changes were also noted in the outer zone of the outer layer of the oocyst wall. These differences could result from the mechanical and chemical treatments used for processing the faeces.