In the neocortex of the reeler mutant mouse, there is inversion in the normal relative positions of polymorphic and pyramidal cells and of large with respect to medium-sized and small pyramidal cells. Granule cells are concentrated at a near-normal mid-cortical level in the mutant. As in the normal animal, and despite cell malposition in reeler, the principal tangential fiber system lies in the zone of polymorphic cells. Large fiber fascicles, known from experimental studies to be principally thalamo-cortical afferents, enter the tangential fiber system in the polymorphic cell zone of both reeler and normal neocortex. In the mutant these fascicles must traverse the full width of the cortex to reach this fiber system in its superficial location. In both normal and mutant animals single fibers, again principally thalamo-cortical afferents, pass from the principal tangertial fiber system to ramify in a fiber feltwork in the zone of granule cells. In the mutant these descend whereas in the normal animal they ascend. Also, as in the normal mouse, single fibers pass radially between all levels of the mutant cortex and the central white matter. Regional variations in the character, the pattern of distribution and the relative prominence of homologous cell and fiber elements are closely parallel in reeler and normal. This suggests that cell differentiation and the tangenital organization of reeler neocortex are normal despite cell malposition in the mutant.