The Drosophila imaginal cells that produce epidermal hairs, the shafts of sensory bristles and the lateral extensions of the arista are attractive model systems for studying the morphogenesis of polarized cell extensions. We now report the identification and characterization of furry, an essential Drosophila gene that is involved in maintaining the integrity of these cellular extensions during morphogenesis. Mutations in furry result in the formation of branched arista laterals, branched bristles and a strong multiple hair cell phenotype that consists of clusters of epidermal hairs and branched hairs. By following the morphogenesis of arista laterals in pupae, we have determined that the branched laterals are due to the splitting of individual laterals during elongation. In genetic mosaics furry was found to act cell autonomously in the wing. The phenotypes of double mutant cells argue that furry functions independently of the frizzled planar polarity pathway and that it probably functions in the same pathway as the tricornered gene. We used a P-element insertion allele as a tag to clone the furry gene and found it to be a large and complicated gene that encodes a pair of large conserved proteins of unknown biochemical function.