Sound transduction in the cochlea depends on the unique high concentrations of K(+) in the endolymph. The production and maintenance of high K(+) concentrations are accompanied by Cl(-) cycling. In this study, we report on an investigation of the expression and localization of TMEM16A/anoctamin 1 (ANO1), a recently cloned Ca(2+)-activated Cl(-) channel, in the mouse cochlea by Western blot and immunhistochemistry. The ANO1 protein was identified in the cochlea by Western blotting. The immunoreactivity was found in stria vascularis as a line and in the organ of Corti as three plaques. The cellular localization of ANO1 was examined by means of double-labeling experiments with anti-claudin 11, a marker for basal cells of the stria vascularis. The results demonstrated that ANO1 colocalized with claudin 11, indicating its expression in basal cells. We also examined ANO1 localization in the organ of Corti by double- and triple-labeling techniques with anti-myosin VI, a marker for hair cells, and anti-synaptophysin, a marker for olivocochlear efferent nerve endings under hair cells. The results clearly showed that ANO1 is colocalized with synaptophysin, but not with myosin VI, indicating that ANO1 is localized at medial olivocochlear efferent nerve endings under outer hair cells. These results suggest that ANO1 may be specifically involved in synaptic transmission from medial olivocochlear efferent nerve endings to outer hair cells in the organ of Corti, as well as Cl(-) cycling in basal cells of the stria vascularis.