To survey the emergence and onset of differentiation of the astrocytic lineage in the developing mouse cerebral wall, the promoter activity of a 2.5 kb 5''-flanking region of glial fibrillary acidic protein (GFAP) was measured in individual developing brain cells using a retrovirus-mediated gene transfer system. We identified precursors for astrocytes in primary culture of embryonic mouse cerebral wall cells by detection of GFAP promoter activity, which was detected approximately 3 days prior to the appearance of GFAP immunoreactivity. Since retroviruses only integrate into the chromosomes of actively proliferating cells, cells detected by this method should have been mitotically active at the time of retroviral infection on day 15 postfertilization (E15). Furthermore, we observed that cells activating GFAP promoter were located near the ventricular surface of cultured cerebral wall slices as a cluster of spherical cells. These results demonstrate that precursor cells for astrocytes exist within the germinative zone of developing cerebral wall, and that these cells are mitotically active on day E15, which is a late stage of neuronal production period in the mouse cerebral wall. The morphology, location and mitotic activity of these cells suggest that they are unlikely to be cells that have been transformed from radial glial cells.