In the Drosophila leg, activation of Notch leads to the establishment of the joints that subdivide the appendage into segments. We find that mutations in bowl result in similar phenotypes to Notch, causing fusion and truncations of tarsal segments (tarsomeres) and, like its close relative Odd-skipped, Bowl is produced in response to Notch signalling at a subset of segment boundaries. However, despite the fact that bowl mutant clones result in fusion of tarsomeres, Bowl protein is only found at the t1/tibial and t5/pretarsal boundaries, not at tarsomere joints. One hypothesis to reconcile these data is that bowl has a role at an earlier stage in tarsal development. We therefore investigated the effects of bowl mutations on the expression of leg 'gap' genes that confer regional identity on the developing leg. Several of these genes have altered expression in bowl mutant cells. For example, bric-a-brac2 is normally expressed in the central part of the tarsus domain but expands into distal and proximal regions in bowl clones. Conversely, ectopic bowl leads to a reduction in bric-a-brac2, with a concomitant expansion of proximal (t1) and distal (t5) tarsomere fates. The bowl gene is therefore required for the elaboration of pattern in the tarsus and its effects suggest a progressive model for the determination of P/D identities. This mechanism might be important in the diversification of arthropod limbs, because it explains how segmented tarsomeres could have arisen from an ancestral limb with an unsegmented tarsus.