The Rac and Cdc42 GTPases share several regulators and effectors, yet perform distinct biological functions. The factors determining such specificity in vivo have not been identified. In a mutational screen in Drosophila to identify Rac-specific signaling components, we isolated 11 alleles of myoblast city (mbc). mbc mutant embryos exhibit defects in dorsal closure, myogenesis, and neural development. DOCK180, the mammalian homolog of Mbc, associates with Rac, but not Cdc42, in a nucleotide-independent manner. These results suggest that Mbc is a specific upstream regulator of Rac activity that mediates several morphogenetic processes in Drosophila embryogenesis.