We have shown by in vitro development of staged follicles in the presence of tritiated proline, as well as by immunoblot analysis, that the Dvs19 and DVS15 chorion proteins of Drosophila virilis are synthesized by the follicular epithelium and are incorporated in the chorion at the terminal stages of choriogenesis. Light microscopy immunolocalization has revealed that these two proteins represent structural components of the endochorion in all the specialized regions of the eggshell, while, contrary to their temporal pattern of secretion, electron microscopic immunolocalization has shown that these two proteins participate in endochorionic structures formed before the onset of their synthesis, thus suggesting an intercalation process during chorion formation. Furthermore, double immunolocalizations demonstrated that cosecreted proteins do not coexist in all secretory vesicles of the follicle cells, indicating that probably the final association of these chorionic components occurs extracellularly. On the basis of these data and those presented in the accompanying paper [Trougakos, I.P., and Margaritis, L.H. (1998) Immunolocalization of the temporally "early" secreted major structural chorion proteins, Dvs38 and Dvs36, in the eggshell layers and regions of Drosophila virilis, J. Struct. Biol. 123, 000-000.], a model for chorion assembly in Drosophilidae is proposed.