The wingless (wg) and decapentaplegic (dpp) genes of Drosophila encode homologs of secreted growth factors and are required for the correct patterning of the appendages. We show that the presumptive tips of both the leg and wing, the distal extreme of the proximodistal axis, are characterized by the close association of cells expressing wg, dpp, and the homeobox gene aristaless (al). Ectopic expression of wg can induce both ectopic al expression and a duplication of the proximodistal axis (the development of supernumerary legs), but only in regions expressing high levels of dpp. Ectopic al expression can induce a duplication of the proximodistal axis in the wing, suggesting that it may be directly involved in axis specification. The proximodistal axis may be specified via a mechanism involving a direct interaction between cells expressing wg, dpp, and possibly al.