We have analyzed several aspects of the development of flies carrying mutations at the Glued locus. Optic lobe abnormalities in individuals heterozygous for the original Glued allele were previously shown to result from an action of this mutation in the retinula cells. We have estimated when the functioning of this gene or its product is required for normal visual system development by using genetic mosaicism induced by somatic recombination and temperature shifts of a temperature-sensitive mutation at this locus. Both methods point to a period in the mid-third instar, suggesting that early events in the formation of ommatidia and/or late events in the program of retinal cells are affected. Application of a new histological stain for developing axons indicates that individuals heterozygous for Glued exhibit abnormalities in the retinula fiber projection by the late third instar. Thus, the adult phenotype is not solely the result of later cellular degeneration or rearrangement. Beneath M+ Gl+ clones which encompass the entire eye were found optic lobe abnormalities with features not seen in either other mosaics or Gl heterozygotes. The possibility that these abnormalities result from temporal asynchrony in the development of eye and and optic lobe in these individuals is discussed and the results of attempts to test this hypothesis are presented.