The cerebellar rhombic lip and telencephalic cortical hem are dorsally located germinal zones which contribute substantially to neuronal diversity in the CNS, but the mechanisms that drive neurogenesis within these zones are ill defined. Using genetic fate mapping in wild-type and Lmx1a −/− mice, we demonstrate that Lmx1a is a critical regulator of cell-fate decisions within both these germinal zones. In the developing cerebellum, Lmx1a is expressed in the roof plate, where it is required to segregate the roof plate lineage from neuronal rhombic lip derivatives. In addition, Lmx1a is expressed in a subset of rhombic lip progenitors which produce granule cells that are predominantly restricted to the cerebellar posterior vermis. In the absence of Lmx1a , these cells precociously exit the rhombic lip and overmigrate into the anterior vermis. This overmigration is associated with premature regression of the rhombic lip and posterior vermis hypoplasia in Lmx1a −/− mice. These data reveal molecular organization of the cerebellar rhombic lip and introduce Lmx1a as an important regulator of rhombic lip cell-fate decisions, which are critical for maintenance of the entire rhombic lip and normal cerebellar morphogenesis. In the developing telencephalon Lmx1a is expressed in the cortical hem, and in its absence cortical hem progenitors contribute excessively to the adjacent hippocampus instead of producing Cajal-Retzius neurons. Thus, Lmx1a activity is critical for proper production of cells originating from both the cerebellar rhombic lip and the telencephalic cortical hem.