In the adult cerebral cortex, many retrovirally labelled clones are widely dispersed, though the mechanisms of this dispersion are not well understood. Here we investigate the temporal sequence of clonal dispersion after labelling progenitors of rat cortical cells with replication-incompetent retroviruses at early stages of cortical neurogenesis, 14-15 days after conception (E14/15). The location of labelled daughter cells was determined 3, 6 or 10 days later. Labelled sibling cells were radially arrayed three days after infection (E18). In contrast, by six days after infection (E20/21), 43% of cortical clones were dispersed non-radially by at least 500 microns. Four of these widespread clones were dispersed longitudinally by > or = 2 mm, implying sustained rates of dispersion of > 15 microns per hour. Dispersed sibling cells occurred within proliferative zones of the forebrain in 35% of widely dispersed clones, suggesting that some dispersion reflects movement of dividing cells. Some clones dispersed beyond the neocortex into the olfactory bulb. Progenitor cell dispersion represents a previously unrecognized mode of migration by which sibling cells become widely dispersed in the developing forebrain.