The Drosophila eye has contributed much to our knowledge of cell differentiation. This understanding has primarily come from the study of the R7 photoreceptor; much less is known about the development of the other classes of photoreceptor or the nonneuronal cone or pigment cells. I have used a dominant-negative form of the Drosophila epidermal growth factor receptor (DER) to show that this receptor tyrosine kinase (RTK) is required for the differentiation of all these cell types, and I have also shown that DER is sufficient to trigger their development. DER is even required in R7, where it can replace Sevenless, another RTK. These results broaden our view of eye development to include the whole ommatidium and suggest that reiterative activation of DER is critical for triggering the differentiation of all cell types.