The use of X-ray-induced mitotic recombination to genetically mark individual cells and their descendants during development has led to the discovery of lineage restriction boundaries in Drosophila imaginal disks, dividing the disks into areas called compartments. Clones of cells initiated after a given developmental stage are unable to grow across these boundaries, even if provided with a growth rate advantage over the remaining cells. It has been suggested that cells within compartments are distinguished by the differential activation of selector genes and that the lineage restrictions are maintained by adhesivity differences between the cells in different compartments, but other mechanisms have not been ruled out. Recently a discrete population of cells with unusual permeability properties has been described along an intersegmental lineage restriction boundary in Oncopeltus, suggesting that a lineage restriction could be maintained by a zone of cells which present a barrier to clone growth. Here we demonstrate by autoradiography the presence of a narrow zone of non-proliferating cells (ZNC) coincident with the presumptive wing margin in the Drosophila wing disk, and suggest that this could account for the observed lineage restriction between presumptive dorsal and ventral surfaces of the wing. As the anterior/posterior compartment boundary does not coincide with a ZNC, the results indicate that different lineage boundaries may be maintained by different mechanisms.