The alpha9 acetylcholine receptor (alpha9 AChR) is specifically expressed in hair cells of the inner ear and is believed to be involved in synaptic transmission between efferent nerves and hair cells. Using a recently developed method, we modified a bacterial artificial chromosome containing the mouse alpha9 AChR gene with a reporter gene encoding green fluorescent protein (GFP) to generate transgenic mice. GFP expression in transgenic mice recapitulated the known temporal and spatial expression of alpha9 AChR. However, we observed previously unidentified dynamic changes in alpha9 AChR expression in cochlear and vestibular sensory epithelia during neonatal development. In the cochlea, inner hair cells persistently expressed high levels of alpha9 AChR in both the apical and middle turns, whereas both outer and inner hair cells displayed dynamic changes of alpha9 AChR expression in the basal turn. In the utricle, we observed high levels of alpha9 AChR expression in the striolar region during early neonatal development and high levels of alpha9 AChR in the extrastriolar region in adult mice. Further, simultaneous visualization of efferent innervation and alpha9 AChR expression showed that dynamic expression of alpha9 AChR in developing hair cells was independent of efferent contacts. We propose that alpha9 AChR expression in developing auditory and vestibular sensory epithelia correlates with maturation of hair cells and is hair-cell autonomous.