uncoordinated (unc) mutants of Drosophila, which lack transduction in ciliated mechanosensory neurons, do not produce motile sperm. Both sensory and spermatogenesis defects are associated with disrupted ciliary structures: mutant sensory neurons have truncated cilia, and sensory neurons and spermatids show defects in axoneme ultrastructure. uncencodes a novel protein with coiled-coil segments and a LisH motif, which is expressed in type I sensory neurons and in the male germline – the only ciliogenic cells in the fly. A functional UNC-GFP fusion protein specifically localizes to both basal bodies in differentiating sensory neurons. In premeiotic spermatocytes it localizes to all four centrioles in early G2,remaining associated with them through meiosis and as they become the basal bodies for the elongating spermatid flagella. UNC is thus specifically required for normal ciliogenesis. Its localization is an early marker for the centriole-basal body transition, a central but enigmatic event in eukaryotic cell differentiation.