Signalling by the epidermal growth factor receptor (EGFR) plays a critical role in the segmental patterning of the ventral larval cuticle in Drosophila: by expressing a dominant-negative EGFR molecule or Spitz, an activating ligand of EGFR, we show that EGFR signalling specifies the anterior denticles in each segment of the larval abdomen. We provide evidence that these denticles derive from a segmental zone of embryonic cells in which EGFR signalling activity is maximal. Within each segment, there is a competition between the denticle fate specified by EGFR signalling and the naked cuticle fate specified by Wingless signalling. The final pattern of the denticle belts is the product of this antagonism between the two signalling pathways. Finally, we show that the segmental zones of high EGFR signalling activity depend on bithorax gene function and that they account for the main difference in shape between abdominal and thoracic denticle belts.