Numerous studies have revealed abnormal cytoarchi-tectonics in the reeler mouse brain. In the present study, acetylcholinesterase (AChE) histochemistry has revealed a distinctive mosaic within the occipital cortex of the reeler mouse. The mosaic does not appear until after the second postnatal week, perhaps in association with eye opening. AChE staining in the visual cortex of normal littermates does not exhibit a mosaic pattern, but rather, is present within bands or laminae. The AChE mosaic in reeler persists into adulthood. Immunocyto-chemical staining of the tenascin glycoprotein, an astrocyte-derived extracellular matrix molecule that is concentrated in boundaries around emerging functional patterns in the CNS, reveals a boundary-mosaic pattern in the first postnatal week. Dil axonal tracing in normal versus reeler mice indicates that the thalamocortical projections may also be associated with the AChE mosaic. The observation that a mosaic is unique to the occipital cortex of reeler mice suggests that it may evolve through abnormal cell and molecular interactions in the mutant cortex that normally lead to the development of functional visual representations.