Cytogenesis in the basilar papilla sensory epithelium of the chicken was investigated through pulse labeling of proliferative cells. Tritiated-thymidine was injected intravenously in chick embryos cultured in petri dishes. All embryos received the injection on the seventh day of incubation (E7), when the progenitors of hair cells and supporting cells are replicating deoxyribonucleic acid (DNA). Cells that were in the synthesis phase of the cell cycle, either at the time of the 3H-thymidine pulse or within 2 hours, incorporated detectable levels of the radioactive DNA precursor. Labeled cells were identified in cochleae from embryos fixed at 0.5, 1, 2, 4, 6, 12 hours, 6 and 8 days after the pulse. One hour after the injection the majority of labeled nuclei were in the basal and middle strata of the sensory epithelium. Four to 6 hours after the injection, a greater number of labeled cells appeared in the lumenal stratum. The patterns of labeled cells in embryos fixed immediately after the injection of 3H-thymidine and in others fixed 6 to 8 days after the injection were unchanged, suggesting that the progenitor cells divide and their progeny differentiate in the sensory epithelium without appreciable transverse migration. Mitotic figures were usually observed only in the lumenal stratum. Analysis of DNA content in the populations of Feulgen-stained nuclei at three levels of depth through the epithelium also produced results consistent with the conclusion that vertical nuclear migration occurs during development of the cells in this sensory epithelium.