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We used colloidal gold probes and post-embedding immunoelectron microscopy to localize circumsporozoite (CS) antigen in 5- and 8-day-old in vitro cultures of Plasmodium cynomologi exoerythrocytic (EE) schizonts. Both small uninucleated and large multinucleated EE schizonts were found in 5-day-old cultures. A mouse monoclonal antibody to the repeat region of the P. cynomolgi CS protein densely labeled the plasma membrane and surface of 5-day-old EE schizonts as well as the surrounding parasitophorous vacuole membrane and space. Density of labeling decreased significantly as EE schizonts increased in size and maturity. Labeling of large, multinucleated 5-day-old schizonts was sparse and limited to the surface of EE schizonts and to small patches of electron dense material which were attached to the inner surface of the parasitophorous vacuole membrane. Mature 8-day-old EE schizonts with developing merozoites had little detectable labeling. CS antigen was not associated with internal structures within developing schizonts. Labeling was not observed in the host cell cytoplasm or on the surface of infected hepatocytes. These findings indicate that epitopes associated with the repeat region of the P. cynomologi circumsporozoite protein are sequestered within infected host cells during EE development.