The extracellular signal-regulated kinases (ERKs) comprise a class of protein-serine/threonine kinases that are activated in response to a wide variety of extracellular signals transduced via receptor tyrosine kinases. Activation of the ERKs requires both threonine and tyrosine phosphorylation suggestive of a key role in mediating intracellular events in response to extracellular cues. To critically assess the role of ERKs in intracellular signaling, a genetically tractable receptor tyrosine kinase system would be invaluable. In this paper we report the identification of a Drosophila homolog of ERK1 and -2, designated DmERK-A. DmERK-A is 80% identical to rat ERK1 and -2 and is rapidly phosphorylated on tyrosine in response to an extracellular signal activating a receptor tyrosine kinase. Biochemical and histological studies reveal its expression in the eye imaginal disc. These studies provide a first step in a genetic analysis of ERK function.