While it is well established that Semaphorin family proteins function as axon guidance ligands in invertebrates and vertebrates, several recent studies indicate that theDrosophilaSemaphorin-1a (Sema1a), a transmembrane Semaphorin, can also function as a receptor during neural development. The regulator of Sema1a reverse signaling, however, remains unknown. In this study, we show that like Sema1a, the well known Semaphorin receptor Plexin A (PlexA), is required for the proper guidance of photoreceptor (R cell) axons in theDrosophilavisual system. Loss ofPlexA, like loss ofsemala, disrupted the association of R-cell growth cones in the optic lobe. Conversely, overexpression ofPlexA, like overexpression ofsema1a, induced the hyperfasciculation of R-cell axons. Unlike Sema1a, however, the cytoplasmic domain of PlexA is dispensable. Epistasis analysis suggests thatPlexAfunctions upstream ofsemala.AndPlexAandsema1ainteract genetically withRho1.We propose that PlexA regulates Semala reverse signaling in theDrosophilavisual system.