We have used immunocytochemistry and in situ hybridization to examine the distribution of neuronal intermediate filament proteins and their mRNAs in the developing mouse cerebellum. First, we demonstrate that alpha-internexin is abundantly expressed in the developing cerebellum and is the only neuronal intermediate filament protein expressed in developing, including migrating, granule neurons. Second, in granule neuron reaggregates in vitro, alpha-internexin is the only neuronal intermediate filament protein highly expressed in the processes of the cultured granule neurons. This in vitro observation is consistent with results from immunocytochemistry and in situ hybridization studies of developing granule neurons in vivo, which suggest that alpha-internexin is the major neuronal intermediate filament protein in developing granule neurons. Finally, the neurofilament triplet proteins are expressed later, and coexist with alpha-internexin in other cells, including Purkinje cells and interneurons in the mature mouse cerebellum. These changes in neuronal intermediate filament composition may regulate neuronal maturation and axonal stability in cerebellar development. Furthermore, alpha-internexin may play a key role in neurite outgrowth and the establishment of neuronal cytoarchitecture.