The Abl family nonreceptor tyrosine kinases, consisting of closely related Abl and Arg (Abl-related gene), play essential roles in mouse neurulation, but their functions in the subsequent development of CNS are poorly understood. Here, we show that conditional deletion of Abl in precursors of neurons and glia on an Arg knock-out background leads to striking cerebellar malformations, including defects in anterior cerebellar morphogenesis, granule cell ectopia, and hypoplasia. Time course analyses reveal that the abnormal anterior cerebellar foliation results from local disruptions of the basement membrane (BM) located between radial glial endfeet and the meninges during embryonic cerebellar development. Granule cell ectopia and hypoplasia are also associated with the breaches in the BM and abnormal Bergmann glial networks during postnatal cerebellar development. In vitro culture experiments indicate that Abl/Arg-deficient granule cells can interact with glial processes and proliferate normally in response to sonic hedgehog compared to cells isolated from control mice. Consistent with these findings, selective ablation of Abl family kinases in cerebellar granule cells alone does not cause any abnormality, suggesting that deletion of Abl/Arg from glia is likely required for the mutant phenotype. Together, these results provide compelling evidence that Abl and Arg play key redundant roles in BM maintenance and cortical lamination in the cerebellum.