Formation and function of the A/P compartment boundary of the Drosophila wing have been studied intensely. The boundary prevents mingling of A and P cells, is characterized by an expression discontinuity of several genes like engrailed, Cubitus interruptus, hedgehog and decapentaplegic and is essential for patterning the wing. Compared with segmental or compartmental boundaries in several other systems which generally manifest as folds or clefts, the wing A/P boundary is morphologically inconspicuous in both the larval and adult stage. We show here that the Drosophila wing A/P boundary, too, is susceptible to fold and cleft formation and that these processes are suppressed by the T-box transcription factor Optomotor-blind (Omb). Using a targeted deletion encompassing the omb wing enhancer [Sivasankaran, R., Vigano, M.A., Muller, B., Affolter, M., Basler, K., 2000. Direct transcriptional control of the Dpp target omb by the DNA binding protein Brinker. EMBO J. 19, 6162-6172] and omb-RNAi to strongly and specifically reduce Omb, we show that omb is required in P cells to prevent aberrant apical fold formation at the A/P boundary of the larval wing disc and cleft formation in the adult wing blade. In the larval A/P fold, DE-cadherin-based adherens junctions appeared intact but the apical microtubule web was strongly reduced.