A correlative light microscopic, ultrastructural, and immunocytochemical study was made of the brains of C57BL/6J mice obtained between embryonic day (E-) 11 to postnatal day (P-) 3. The deployment of radially oriented glial cells within the neopallium was already apparent by E-12, at which time the expanded endfeet of processes abutting the basement membrane at the pial surface showed ultrastructural evidence of glial differentiation. Scattered, horizontally arranged glial cells were also observed within the marginal zone prior to the arrival of the cortical plate neurons. Indirect immunofluorescence for glial fibrillary acidic protein (GFAP) was detected at the outer margins of the neopallium at E-12, antedating the expression of GFAP in radially organized fibers. Radial fibers traversing the full thickness of the neopallium became strongly immunoreactive for GFAP only at and after day E-16. These findings provide evidence of early gliogenesis in the developing mouse forebrain and indicate that GFAP is as readily demonstrable in mouse astrocytes and radial glial fibers as it is in those of man and subhuman primates.