In the chicken cochlea, the structural features of the cilia bundles of individual hair cells vary systematically along the length of the sensory epithelium. As a first approach to understanding the developmental mechanisms that underlie this precise arrangement of structurally distinct hair cells, the spatiotemporal pattern of the terminal mitoses of their precursor cells was investigated by administering 3H-thymidine, a radioactive precursor to DNA. This demonstrated that the first hair cells were produced during the sixth day of incubation and formed a longitudinal band that extended along most of the length of the sensory epithelium. The epithelium grew further through appositional addition of hair cells at the edges of this first band of cells, and the hair cell addition process expanded into the surrounding areas during the next 3 days. By the ninth day of incubation all the hair cells in the sensory epithelium except for those at the peripheral edges in the distal (apex) portion had been produced through terminal mitoses. Our results have demonstrated that hair cells that have similar stereocilia phenotypes do not all leave the mitotic cycle at the same time.