Embryonic and young postnatal mice were exposed once to thymidine-H³, to label cells preparing for division. Histogenesis of cerebellum was studied in autoradiograms. Purkinje cells and neurons of roof nuclei form simultaneously in the primitive ependyma of the young embryo and migrate outward to reach their final positions. A transient external granular layer arises by proliferation of cells on the lateral caudal cerebellar surface lining the fourth ventricle. These cells migrate over the external cerebellar surface, and continue to proliferate abundantly until a few weeks after birth. The layer disappears in the third postnatal week. Most external granule cells divide a few times and then migrate inward past the Purkinje cells to become granule cell neurons. A smaller population of cells arises in the granule layer itself and is characterized by an unusually long interval between deoxyribonucleic acid (DNA) synthesis and mitosis. These cells migrate short distances and may assume positions around the perikarya of Purkinje cells; cells of this class are probably neuroglia. An hypothesis is presented which accounts for the extensive cell migrations during histogenesis as a means for attaining particular synaptic contacts.