The inward migration of external granule cells (EGC) from the pial surface of the developing cerebellum to form the (internal) granule cell layer was examined using SEM. Cerebella from male mice ranging in age from days 1-20 were fixed, then fractured through the developing pyramid region. EGC were initially unspecialized cells, forming 2-3 layers at the pial surface. EGC layers increased to 6-8, granule cells in the deeper regions elongated, and a prominent space formed between superficial and deep (premigratory) strata. During peak migration (days 8-12), nests of 4-6 EGC were associated with Bergmann glial fibers (BF) of the Golgi epithelial cells, which crossed molecular and EGC layers to terminate as spiny endfeet at the pial surface. Fibrils of extracellular material (ECM) often linked both premigratory and migrating EGC with a nearby BF. The molecular layer thickened considerably and the parallel fibers were traversed by an increasing number of Bergmann fibers and Purkinje cell processes during this period. As active migration slowed (days 13-20) and EGC reached their destination below the Purkinje cell layer, they lost their polarity and were enmeshed in ECM. The role of the Bergmann fibers and extracellular material in granule cell migration is considered.