Genetic mosaic studies indicate that environmental cues play a critical role in photoreceptor cell (R-cell) development in the Drosophila compound eye. Recent analysis of the sevenless gene suggests that its product, a cell surface protein containing a putative intracellular domain homologous to tryosine kinases, is a receptor for a signal specifying an R7-specific pathway of cellular development. We report the identification of a second genetic locus, bride of sevenless (boss) required for the development of R7. Genetic mosaic analysis demonstrates that boss expression is required in the R8 cell for a neighboring cell to become R7. However, the development of R1-R6 and R8 is boss-independent. These data indicate a central role for R8 in directing ommatidial assembly and are consistent with boss encoding the ligand recognized by the sevenless receptor.