Birds respond to hair cell loss by stimulating cell division in the otherwise mitotically quiescent sensory epithelium and by generating new hair cells. We examined cell proliferation during hair cell regeneration in chick basilar papilla by using 5-bromo-2'-deoxyuridine (BrdU). Chicks were noise exposed for 4 or 24 hours and injected with BrdU, and cochleae were immunohistochemically labeled to detect BrdU. Immunoreactivity after short-term postinjection survival identified when cells entered S phase. For both 4 and 24 hour exposures, cells in S phase were first detected in the sensory epithelium after an injection at 18 hours after the onset of exposure and were also present after injections at 24, 30, 36, 42, 48, 72, 96, 120, and 144 hours. The most cells in S (or G2) phase were detected at 42 and 72 hours for 24 hour exposures and at 48 hours for 4 hour exposures. Chicks that survived for long periods after injection had BrdU-labeled hair cells, indicating that precursor cells that divided in the presence of BrdU generated new hair cells. Moreover, labeled hair cells and supporting cells were grouped into discrete clusters, suggesting that cells within each cluster are clonally related. Support for this hypothesis was provided by experiments showing that the number of labeled cells increased when chicks survived for longer periods after a single BrdU injection. These findings suggest that progenitors within the sensory epithelium may undergo several rounds of division to generate the appropriate number of new hair cells and supporting cells.