To thoroughly study developmental problems it is often desirable to identify specific cells at the resolution of the electron microscope (TEM). Specific antibodies, and immunogold and other antibody labelling techniques can be successfully used with the TEM. But for these techniques to be successful there must be substantial adjustments for each antibody and tissue analyzed. To develop a more generally applicable labelling method we took advantage of the enhancer trap technique in Drosophila. Enhancer trap fly strains show cell- and/or tissue-specific beta-galactosidase expression which can be visualized by a simple X-gal staining procedure. To combine the power of the enhancer trap approach with electron microscopy, we have improved the fixation and staining conditions, which allow detection of X-gal crystals (by TEM) and thus provide precise information on ultrastructural morphology. We have tested our technique using the well-known midline glial cells and examined these cells between late embryonic and pupal developmental stages. The four embryonic midline glial cells found in each neuromere reside ventrally and dorsally to the midline of the neuropile and are closely associated with unpaired neurons, major commissures, and other types of glial cells. During larval and pupal life dramatic cell growth and endomitotic nuclear replication occur in midline glial cells. By the end of larval life, the giant midline glial cells fragment to give rise to a variable number of small midline glial cells. Here we show that the combination of transmission electron microscopy with cytochemical detection of beta-galactosidase expression represents a promising and valuable tool for the study of the morphology and development of specific cell types.