Infective stages of Toxoplasma and Sarcocystis have been studied by the freeze fracture technique. The outer membrane of the pellicle is continuous and shows an apical 8 + 1 particles rosette in the P fracture face. The inner membrane complex is made of rectangular flattened vesicles aligned in longitudinal rows and joined in a puzzle like fashion. Sarcocystis has 11 of these rows whereas their number varies in Toxoplasma. A posterior interruption is present. Anteriorly is one truncated conical plate apically opened by a vertical ridge. The membranes of the inner complex are characterized by parallel alignment of particles (in P faces) some of which are joined and continuous with double rows radiating in the apical cap. Those rows correspond in number and arrangement with the underlying microtubules. The rhoptries membranes show periodic circular arrays of particles.