The sources of new hair cells, support cells, and Schwann cells were identified in the statoacoustic end organs of normal post-embryonic fish (Astronotus ocellatus). S-phase cells, defined as cells that take up 3H-thymidine in preparation for mitosis, and their progeny were visualized using autoradiography. Two types of S-phase cells were found: 'embryonic-like neuroepithelial' (NE) cells and 'basally located S-phase' (BLS) cell. The NE cells had elongated nuclei and processes extending basally and apically. Thirty minutes after the thymidine injection labeled NE cell nuclei were found between hair cell nuclei and support cell nuclei. After two and four hours survival some labeled NE nuclei were closer to the lumen, where they divided. One labeled support cell was seen after four hours survival and one labeled hair cell after nine hours survival. Significant numbers of labeled support and hair cells were not seen until 24 h survival. These results led us to identify NE cells as the immediate source of new hair cells and support cells. The BLS cells had small nuclei with little surrounding cytoplasm. Shortly after the thymidine injection BLS cells were found between the hair cell nuclei and the basement membrane, and they underwent mitosis in this position. The BLS cells resemble intra-epithelial Schwann cells, except that they are not curved around axonal profiles. We suggest that the BLS cells are Schwann cell precursors.