The morphology of the cochlear nuclei of normal and reeler mutant mice were studied in Nissl-stained sections. The cochlear nucleus in both mice is divisible into three parts: the anteroventral, posteroventral, and dorsal nuclei. Nine cell types can be recognized in the normal mouse. In the anteroventral nucleus spherical cells occupy the rostral pole. Globular cells are located caudally and extend to the interstitial region of the anteroventral nucleus. In the posteroventral nucleus multipolar cells are located rostrally and dark-staining cells occupy the caudal pole. Multipolar cells are also present in the anteroventral nucleus and in the deep region and molecular layer of the dorsal cochlear nucleus. The dorsal and lateral aspects of the ventral nuclei are covered by a granule cell layer. The dorsal nucleus consists of superficial molecular and pyramidal layers and a deep region. The deep region contains small and giant cells as well as multipolar cells. The pyramidal layer is made up of pyramidal cells, horizontal cells, and granule cells. Small cells are also present in the molecular layer and throughout the ventral nuclei. The dorsal cochlear nucleus of the reeler mutant mouse is disorganized and the molecular layer is reduced in thickness. The organization of the pyramidal layer is disrupted with granule cells superficial to pyramidal and horizontal cells. Cells which appear to be homologous to pyramidal cells are also present in the deep region of the dorsal nucleus. The total number of granule cells is reduced by an average of 42% over the whole nucleus and the reduction in granule cells is greatest in the granule cell cap covering the dorsal and lateral surface of the ventral cochlear nuclei. The cytoarchitecture of the ventral cochlear nucleus appears normal.