Immunocytochemical staining with antisera directed against glial fibrillary acidic protein (GFA) was used to examine the cellular composition of the proliferative ventricular zone (VZ) in the occipital lobe of rhesus monkey fetuses during the first half of their 165-day gestational period. Electron microscopic analysis revealed a small number of GFA-positive cells in specimens of embryonic (E) ages E39 and E40. By E47 about 28% of the cells in the VZ were immunoreactive. This percentage increased to 37% at E61 and reached 60% by E80. The fraction of GFA-positive mitotic figures followed the same general tendency with 34, 47, and 80% being labeled at E47, E61, and E80, respectively. The present results reveal the existence of two basic classes of cells, GFA positive and GFA negative, indicating that in the primate brain, glial and neuronal precursor cells may coexist in the ventricular zone at embryonic ages when few, if any, neurons have become postmitotic.