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The fine organization of the "membrane complex" present at the tip of the epicyte folds in the sporozoan Greguritia trophozoite has been studied. Electron microscopy revealed bundles of intermcdiate filaments (8-10 nm) in the interspace between plasma and middle membrane. Doublets of filaments are present in the cytoplasm adhering to the i n n s membrane. The narrow space between the inner and middle membrane exhibits electron dense dots arranged in pairs. Freeze-fracture replicas reveal a different organization in each of the three membranes. The plasma membrane shows the Pface rich in particles and E-face with a few clusters of intramembrane particles (IhlPs). Parallel ridges are present on the E-face and parallel grooves on the P-face of the intermediate membrane. The P-face of the inner membrane shows double longitudinal arrays of IhlPs and the E-face has large grooves alternating with rows of ridges. The rows of particles on the P-face of the inner membrane are in register with the clusters of filaments seen in the cytoplasm and adhering to the inner membrane.Our observations suggest connections among the three membranes at the tip of the epicyte folds; moreover, an interaction occurs between the intermediate filaments and intramembrane complexes. The arrangement of the membranes is discussed in reference to the gliding of gregames. ACKNOWLEDGEMENTS

Conventional tracsmission electron microscopy (TEM) and freeze-fracture technique have been carried out on the trophozoite of Gregaritta sp. to examine th: ultrastructural modifications of the membrane organization during the sequence of events characterizing exocytosis. Electron microscopical obscr\*ations resealed that exoqtosis is accompanied by the break of the inner and middle membranes, constituents of the three "unit membrane complex", which form the pellicle of this organism, and by their fusion at the site of the successive micropore formation.Freeze fracturing showed internal membrane organization of these complexes. Along the lateral surface of the epicytic folds numerous aggregates of rosette-shaped IhlPs (intrarnembrane particles) weie evident. These are specialized points of the membrane organized to make contact possible between the secretory vesicle and the membrane complex. Freeze-fracture images give also evidence of progressive membrane modifications in relation with the secretory process.