The relative positions of the principal neuronal classes of neocortex are inverted in the reeler mutant mouse. Neurons formed at 48-hourly intervals throughout the period of neocortical cytogenesis between E11 and E17 are labeled by [3H]thymidine. The positions of the labeled cells during and subsequent to their migrations are traced by autoradiography. Simultaneously-formed cohorts reach the neocortex at the same time in normal and reeler animals. After E13, subsequent to the appearance of the cortical plate, cohorts of migrating cells in the normal animal ascend to the interface of the cortical plate and marginal layer where they come to rest in a narrow laminar zone. In reeler, by contrast, migration is arrested in the depths of the cortex. The migrating cell appears unable to ascend through the zone occupied by the preceding cohorts. At the completion of migration neurons of both genotypes become fixed in position and undergo little subsequent shift in their relative positions in the course of future cortical growth. Despite the anomaly of migrations and the post-migratory positions of neurons in reeler, cohorts of cells formed at the same time in the two genotypes give rise to the same neuronal classes.