The growth and intracytoplasmic development of two yellow fever virus strains (wild and French neurotropic) were studied in Aedes aegypti cells (clone C 17). Despite a longer period of latency for the vaccine virus, infected cells appeared similar. The cisternae of the rough endoplasmic reticulum (RER) were swollen and formed vesicles which contained the virus. This RER appeared to be the predominant locus of viral synthesis and maturation. Cytopathic effect appeared when the cells were filled with vesicles, and it was characterized by cell degeneration and lysis.