During testis development, the rapid morphological changes initiated bySryrequire the coordinate integration of many signaling pathways. Based on the established role of the platelet-derived growth factor (PDGF) family of ligands and receptors in migration, proliferation, and differentiation of cells in various organ systems, we have investigated the role of PDGF in testis organogenesis. Analysis of expression patterns and characterization of the gonad phenotype inPdgfr-α−/−embryos identified PDGFR-α as a critical mediator of signaling in the early testis at multiple steps of testis development.Pdgfr-α−/−XY gonads displayed disruptions in the organization of the vasculature and in the partitioning of interstitial and testis cord compartments. Closer examination revealed severe reductions in characteristic XY proliferation, mesonephric cell migration, and fetal Leydig cell differentiation. This work identifies PDGF signaling through the α receptor as an important event downstream ofSryin testis organogenesis and Leydig cell differentiation.