The development of the Drosophila melanogaster wing depends on the correct regulation of cell survival, growth, proliferation, differentiation, and pattern formation. These processes, and the genes controlling then, are common to the development of epithelia in many different organisms. To identify additional genes contributing to wing development we have carried out a genetic screen in mosaic wings carrying clones of homozygous mutant cells. We obtained 12 complementation groups corresponding to genes with a proven role in wing formation such as smoothened, thick veins, mothers against dpp, expanded, and fat and 71 new complementation groups affecting the pattern of veins and the size of wing. We mapped one of these groups to the mediator15 gene (med15), a component of the Mediator complex. We show that Med15 and other members of the Mediator complex are required, among other processes, for the transcription of decapentaplegic target genes.