Reeler, a recessive mutation in mice, causes cytoarchitectonic abnormalities of the cerebral and cerebellar cortices. Although the relative positions of classes of cells in the cerebral cortex are markedly abnormal, the time of origin of cells in each class appears to be normal by autoradiographic criteria. In the retrohippocampal cortical structures and in the adjacent neocortex, the earliest generated cell class, the polymorphic, arises at the same embryonic time in normal and reeler littermates. Whereas these cells come to form the deepest stratum of the normal cortex, they lie in an immediately subpial position in the mutant, there being no cell-sparse external plexiform (molecular) layer. A population of larger cells is generated next; these normally come to lie in more superficial layers but instead lie deep to the polymorphic cells in the mutant. Granule cells of the neocortex arise last, simultaneously in normal and reeler; they form the most superficial neocortical cell stratum in the normal, but lie at a more restricted level within the large cell zone in the mutant. These data indicate that the cytoarchitectonic anomaly in reeler is independent of the spatiotemporal program of cell generation. It appears likely that the reeler genetic locus governs embryonic events, as yet undefined, that direct the postmitotic migrating young neuron to class-specific levels within the cortex.