Eyes absent (eya) encodes a member of a network of nuclear transcription factors that promotes eye development in both vertebrates and invertebrates. Despite extensive studies, the molecular mechanisms whereby cell-cell signaling pathways coordinate the function of this retinal determination gene network remain unknown. Here, we report that Drosophila Eya function is positively regulated by mitogen-activated protein kinase (MAPK)-mediated phosphorylation and that this regulation extends to developmental contexts independent of eye determination. In vivo genetic analyses, together with in vitro kinase assay results, demonstrate that Eya is a substrate for extracellular signal-regulated kinase, the MAPK acting downstream in the receptor tyrosine kinase (RTK) signaling pathway. Thus, phosphorylation of Eya appears to provide a direct regulatory link between the RTK/Ras/MAPK signaling cascade and the retinal determination gene network.