Sections from the cochlear nuclei of normal and reeler mutant mice were treated by the method of Karnovsky and Roots ('64) for the demonstration of cholinesterase activity. In one group of controls ethopropazine was used to inhibit pseudocholinesterases and in another group BW 284C 51 was used to inhibit acetylcholinesterase (AChE). The distribution of AChE-positive fibers in the normal mouse is given. The reeler mutant mouse differs from the normal in that there are no AChE-positive fibers in the dorsal nucleus, very few in the granule cell layer separating the dorsal from ventral nuclei, none in the granule cell layer covering the lateral aspect of the ventral nucleus, and the number in the posteroventral and caudal anteroventral nuclei are greatly reduced. Perisomatic terminals, small and large terminal boutons, and boutons en passant are present in both normal and reeler mutant mice. AChE-positive cell bodies are found in the deep region of the dorsal nucleus, the rostral part of the posteroventral nucleus, and the anteroventral nucleus of both normal and reeler mutant mice. The observations in the normal mouse are compared to the distribution of AChE-positive fibers and the types of terminals formed by noncochlear afferents in other species. Several possible causes of the lesion of the AChE-positive fibers in the reeler mutant mouse are considered.