The molecular basis for patterning of complex organ structures like the lung and insect tracheal system is unknown. Here, we describe the Drosophila gene branchless (bnl ) and demonstrate that it is a key determinant of the tracheal branching pattern. bnl is required for tracheal branching and is expressed dynamically in clusters of cells surrounding the developing tracheal system at each position where a new branch will form and grow out. Localized misexpression of bnl can direct branch formation and outgrowth to new positions. Generalized misexpression activates later programs of tracheal gene expression and branching, resulting in massive networks of branches. bnl encodes a homolog of mammalian fibroblast growth factors (FGFs) and appears to function as a ligand for the breathless receptor tyrosine kinase, an FGF receptor homolog expressed on developing tracheal cells. The results suggest that this FGF pathway specifies the tracheal branching pattern by guiding tracheal cell migration during primary branch formation and then activating later programs of finer branching at the ends of growing primary branches.