The dorsoventral (D/V) lineage boundary in the developing wing disc of Drosophila restricts growing cells to the prospective dorsal or ventral compartments of the wing blade. This restriction appears along the prospective margin of the wing some time during the middle to late stages of wing disc growth. It has been proposed that the restriction is established and maintained by the formation of a zone of non-proliferating cells that acts as a barrier between cells in the dorsal and ventral compartments (O'Brochta and Bryant, Nature 313, 138-141, 1985). In the adult, however, no group of barrier cells has been identified between the compartments. This study will show the following. (1) A group of cells does exist that lies between the dorsal and ventral rows of margin bristle precursors; these cells, which express cut in the late third instar wing disc, are thus in an ideal position to act as barrier cells. (2) This cut-expressing region is split into dorsal and ventral regions by the expression of the dorsal-specific gene apterous. (3) The D/V lineage restriction defined by marked dorsal and ventral clones lies in the middle of the cut-expressing region and is exactly congruent with the boundary of apterous expression. (4) No group of barrier cells is observed between dorsal and ventral clones. (5) Clones often run along the boundary for long distances, suggesting that they can grow along the D/V boundary without crossing it. These results thus do not support the existence of a groups of cells acting as a barrier between dorsal and ventral compartments. Nor do they support a critical role for division rates near the D/V boundary in establishing or maintaining the lineage restriction.