The organ of Corti, which is the sensory organ of hearing, consists of a single row of inner hair cells and three rows of outer hair cells in mice. The auditory hair cells develop from auditory progenitors. Hair cell development is related to several genes, including PTEN. Homozygous null mutant (PTEN(-/-)) mice die at around embryonic day 9, when hair cells are extremely immature. Moreover, in heterozygous PTEN knockout mice, it was found that PTEN regulates the proliferation of auditory progenitors. However, little is known about the molecular mechanism underlying this regulation. In the present study, we generated PTEN conditional knockout in the inner ear of mice and studied the aforementioned molecular mechanisms. Our results showed that PTEN knockout resulted in supernumerary hair cells, increased p-Akt level, and decreased p27(kip1) level. Furthermore, the presence of supernumerary hair cells could be explained by the delayed withdrawal of auditory progenitors from the cell cycle. The increased p-Akt level correlates with p27(kip1) downregulation in the cochlea in the Pax2-PTEN mice. The reduced p27(kip1) could not maintain the auditory progenitors in the nonproliferative state and some progenitors continued to divide. Consequently, additional progenitors differentiated into supernumerary hair cells. We suggest that PTEN regulates p27(kip1) through p-Akt, thereby regulating the proliferation and differentiation of auditory progenitors.