Many mutations in Drosophila melanogaster affect the morphology of the adult compound eye. However, the times at which the phenotypes first become manifest in development are, in most cases, unknown; they can occur at any of a series of stages. Among mutants in which eyes appear externally similar, the developmental stage of onset of each defect may be quite different. Pattern formation in the compound eye begins during the late third larval instar in the eye imaginal disc, when a wave of morphogenesis crosses the disc from posterior to anterior. As this wave crosses the disc, there appears in its wake an array of photoreceptor neuron clusters and accessory cells that will comprise the adult ommatidia. Eye discs from 20 abnormal-eye mutants were analyzed using monoclonal antibodies that highlight various aspects of the developing array, to observe the stage at which each anomaly becomes evident. Some mutations apparently affect precursor cells, others the setting up of the pattern, others maintenance of the pattern, and still others later morphogenetic events.