We have used enhancer trap lines as markers to recognize glial cells in the wing peripheral nervous system of Drosophila melanogaster. Their characterization has enabled us to define certain features of glial differentiation and organization. In order to ask whether glial cells originate within the disc or whether they migrate to the wing nerves from the central nervous system, we used two approaches. In cultured wing discs from glial-specific lines, peripheral glial precursors are already present within the imaginal tissue during the third larval stage. Glial cells differentiate on a wing nerve even in mutants in which that nerve does not connect to the central nervous system. To assess whether peripheral glial cells originate from ectoderm or from mesoderm, we cultured discs from which the mesodermally derived adepithelial cells had been removed. Our findings indicate that peripheral glial cells originate from ectodermally derived cells. As has already been shown for the embryonic central nervous system, gliogenesis in the periphery is an early event during adult development: glial cells, or their precursors, are already present at stages when neurons are still differentiating. Finally, our results also suggest that peripheral glial cells may not display a stereotyped arrangement.