ABSTRACT In many parts of the central nervous system, the elongated processes of radial glial cells are believed to guide immature neurons from the ventricular zone to their sites of differentiation. To study the clonal relationships of radial glia to other neural cell types, we used a recombinant retrovirus to label precursor cells in the chick optic tectum with a heritable marker, the E. coli lacZ gene. The progeny of the infected cells were detected at later stages of development with a histochemical stain for the lacZ gene product. Radial glia were identified in a substantial fraction of clones, and these were studied further. Our main results are the following, (a) Clones containing radial glia frequently contained neurons and/or astrocytes, but usually not other radial glia. Thus, radial glia derive from a multipotential progenitor rather than from a committed radial glial precursor, (b) Production of radial glia continues until at least embryonic day (E) 8, after the peak of neuronal birth is over (∽E5) and after radial migration of immature neurons has begun (E6–7). Radial glial and neuronal lineages do not appear to diverge during this interval, and radial glia are among the last cells that their progenitors produce, (c) As they migrate, many cells are closely apposed to the apical process of their sibling radial glia. Thus, radial glia may frequently guide the migration of their clonal relatives, (d) The population of labelled radial glia declines between E15 and E19–20 (just before hatching), concurrent with a sharp increase in the number of labelled astrocytes. This result suggests that some tectal radial glia transform into astrocytes, as occurs in mammalian cerebral cortex, although others persist after hatching. To reconcile the observations that many radial glia are present early, that radial glia are among the last offspring of a multipotential stem cell, and that most clones contain only a single radial glial cell, we suggest that the stem cell is, or becomes, a radial glial cell.