Nonreceptor protein tyrosine kinases of the Src family regulate the survival, proliferation, differentiation, and motility of many cell types, but their roles in brain development are unclear. Biochemical and in vitro experiments implicate Src and Fyn in the Reelin-dependent tyrosine phosphorylation of Dab1, which controls the positioning of radially migrating neurons in many brain regions. However, genetic evidence that either Src or Fyn mediates Reelin-dependent migrations in vivo has been lacking. Here, we report that, although Src is dispensable and although the absence of Fyn causes an intermediate phenotype, the combined absence of Src and Fyn almost abolishes tyrosine phosphorylation of Dab1 and causes defects in the fetal cortex and cerebellum very similar to those of dab1 mutants of the same age. Neurogenesis is not detectably affected, but the layering of neurons in the cortex is inverted, and the formation of the Purkinje plate is impaired. This implies that Src and Fyn are needed for Reelin-dependent events during brain development.