The present study has used two different approaches for labelling progenitor cells at the optic vesicle stage in order to examine patterns of clonal expansion and cellular dispersion within the developing retina. X-inactivation transgenic mice and chimeric mice expressing the lacZ reporter transgene were examined during development and in adulthood to study the radial and tangential dispersion of proliferating neuroepithelial cells and postmitotic retinal cells of known identities. Chimeric retinas were used to measure tangential dispersion distances, while transgenic retinas were used to assess the frequency of tangential dispersion for individual populations of retinal neurons. Tangential dispersion is shown to be a universal feature of particular retinal cell types, being contrasted with the strictly radial dispersion of other cells. Tangential dispersion is a relatively short-distance phenomenon, with distinct dispersion distances characteristic for cone, horizontal, amacrine and ganglion cells. Embryonic and postnatal retinas show that tangential dispersion occurs at different times for these distinct cell types, associated with their times of differentiation rather than their neurogenetic periods. These developmental results rule out the possibility that tangential dispersion is due to a passive displacement produced by the proliferation of later-born cells, or to the lateral dispersion of a dividing sibling; rather, they are consistent with the hypothesis that tangential dispersion plays a role in the establishment of the orderly spatial distribution of retinal mosaics.