Strict control of cellular proliferation is required to shape the complex structures of the developing embryo. The organ of Corti, the auditory neuroepithelium of the inner ear in mammals, consists of two types of terminally differentiated mechanosensory hair cells and at least four types of supporting cells arrayed precisely along the length of the spiral cochlea. In mice, the progenitors of greater than 80% of both hair cells and supporting cells undergo their terminal division between embryonic day 13 (E13) and E14. As in humans, these cells persist in a non-proliferative state throughout the adult life of the animal. Here we report that the correct timing of cell cycle withdrawal in the developing organ of Corti requires p27(Kip1), a cyclin-dependent kinase inhibitor that functions as an inhibitor of cell cycle progression. p27(Kip1) expression is induced in the primordial organ of Corti between E12 and E14, correlating with the cessation of cell division of the progenitors of the hair cells and supporting cells. In wild-type animals, p27(Kip1) expression is downregulated during subsequent hair cell differentiation, but it persists at high levels in differentiated supporting cells of the mature organ of Corti. In mice with a targeted deletion of the p27(Kip1) gene, proliferation of the sensory cell progenitors continues after E14, leading to the appearance of supernumerary hair cells and supporting cells. In the absence of p27(Kip1), mitotically active cells are still observed in the organ of Corti of postnatal day 6 animals, suggesting that the persistence of p27(Kip1) expression in mature supporting cells may contribute to the maintenance of quiescence in this tissue and, possibly, to its inability to regenerate. Homozygous mutant mice are severely hearing impaired. Thus, p27(Kip1) provides a link between developmental control of cell proliferation and the morphological development of the inner ear.