Monoclonal antibodies (MAbs) and colloidal gold probes were used to localize circumsporozoite (CS) protein and two unrelated polypeptides in developing oocysts and salivary gland sporozoites of the 17X (NL) strain of Plasmodium yoelii. MAbs NYS1, NYS2, and NYS3 recognized different epitopes of the P. yoelii CS protein and produced similar patterns of immunolabelling on developing oocysts and sporozoites. A small percentage of oocysts contained developing sporoblasts and sporozoites that did not exhibit surface reactivity to MAbs NYS1, NYS2 or NYS3, although internal labelling was associated with endoplasmic reticulum (ER). These sporozoites were still capable of completing development and invading salivary glands where they could be found adjacent to sporozoites with densely labelled surface coats. If these sporozoites are infective, their presence may explain in part the failure of CS vaccines to completely protect immunized animals against challenge. The non-CS antigen recognized by MAbs NYS4 did not become abundant until late in sporogony. Some gold labelling was associated with the surface of budding and mature sporozoites, but the antigen was most abundant within the cytoplasm and micronemes. A second non-CS antigen identified by NYS5 first appeared in 7-day-old oocysts, although labelling was sparse. Small quantities of antigen appeared on the sporoblast membrane, cytoplasmic clefts and ER of oocysts and was associated with micronemes and the surface of budding and mature sporozoites. As the role played by non-CS antigens in the biology of the parasite is not yet known, further characterization of their function is needed before their potential as vaccine candidates can be determined.