Gregarines, parasitic protozoa of invertebrates, possess a highly differentiated cell surface, with three cortical membranes and associated structures. Transmission electron microscopy and freeze-fracture reveal the presence of two cytomembranes lying uniformly under the plasma membrane. The density and the distribution of the intramembraneous particles (IMPs) in the plasma membrane of Gregarina blaberae are similar to those reported for other eukaryotic cells. The IMP density is lower in the cytomembranes than in the plasma membrane. The distribution of IMPs in the different fracture faces of the two cytomembranes suggests that they are in topological continuity, forming either side of a flattened vesicle or cisterna. The sizes of the cytomembrane IMPs show a high variability. The nature of the IMPs, both for the plasma membrane and the cytomembrane, is discussed with regard to the integral proteins and glycoproteins of the ghost. The cell surface of G. blaberae exhibits numerous longitudinal folds with three types of cortical membrane-associated structures: 12 nm filaments, an internal lamina, and homogeneous structures described as 'rippled dense structures'. The 12 nm filaments, running under the cytomembranes along the longitudinal axis of each fold, exhibit the properties of intermediate filaments. Their distribution in mature cells and during the growth process suggests a participation in cell surface morphogenesis. The internal lamina, also localized under the cytomembranes, would stabilize each fold and assure a scaffolding function between the numerous folds. The rippled dense structures, settled on the external cytomembrane, show a regular distribution at the top of each fold. The membrane-associated structures are discussed with regard to the gliding movement mechanism.