Regulation of actin filament length and orientation is important in many actin-based cellular processes. This regulation is postulated to occur through the action of actin-binding proteins. Many actin-binding proteins that modify actin in vitro have been identified, but in many cases, it is not known if this activity is physiologically relevant. Capping protein (CP) is an actin-binding protein that has been demonstrated to control filament length in vitro by binding to the barbed ends and preventing the addition or loss of actin monomers. To examine the in vivo role of CP, we have performed a molecular and genetic characterization of the beta subunit of capping protein from Drosophila melanogaster. We have identified mutations in the Drosophila beta subunit-these are the first CP mutations in a multicellular organism, and unlike CP mutations in yeast, they are lethal, causing death during the early larval stage. Adult files that are heterozygous for a pair of weak alleles have a defect in bristle morphology that is correlated to disorganized actin bundles in developing bristles. Our data demonstrate that CP has an essential function during development, and further suggest that CP is required to regulate actin assembly during the development of specialized structures that depend on actin for their morphology.