The adult appendages of Drosophila are formed from imaginal discs, sheets of epithelial cells that proliferate during larval development and differentiate during metamorphosis. wingless (wg, DWnt-1) protein, a putative signaling molecule, is expressed only in prospective ventral cells in each of the leg discs. To test the role of wg, we have generated randomly positioned clones of cells that express wg protein constitutively. Clones that arise in the prospective ventral portions of the leg discs develop normally. In contrast, dorsally situated clones give rise to ventrolateral patterns and exert a ventralizing influence on neighboring wild-type tissue. We propose that wg protein organizes leg pattern along the dorsoventral axis by conferring ventral positional information within the disc.