ABSTRACT Inactivation of the X-linked lacZ transgene provides a novel and powerful way of distinguishing between clonally related cellular populations in X inactivation mosaics. This ability to distinguish between clonal populations of cells in the mature cortex permits inferences to be made about cellular dispersion patterns during cortical development. The present study addresses the extent to which radial and tangential dispersion patterns contribute to different regions of the cerebral cortex by quantifying the extent of cellular mixing between clonally distinct cells in separate domains of the medial, dorsolateral and lateral cortices. We show that stripes running perpendicular to the cortical layers are more likely to be seen in the medial and dorsolateral regions, and that the appearance of a stripe is attributed to about two-thirds of the cells being of the same colour. Both neurons and glia appeared to exhibit the same ratio of cell mixing. In the lateral regions of the cortex, stripes were not apparent, and cell mixing was roughly equal. In the barrel-field region of the somatosensory cortex we looked for a correspondence between cytoarchitectural features and clonal borders but found no correlation. These results demonstrate, first, that although there is widespread radial dispersion, no cortical region is composed of radially arrayed stripes of cells in which all members of a stripe are derived from a single progenitor. Second, they demonstrate that, within regions containing a sizeable fraction of cells that do migrate radially, the boundaries of individual stripes do not always coincide with single anatomical units of cortical specialization, such as individual barrels.