Tight junctions (TJs) are indispensable for the establishment of compositionally distinct fluid compartments in the inner ear, but our knowledge of the claudins, TJ adhesion molecules, in the inner ear is still fragmentary. We examined the expression and distribution of claudin-1 to claudin-18 (except for claudin-7, -13 and -17) in the inner ear by immunofluorescence microscopy. In the cochlea, the organ of Corti expressed claudin-1, -2, -3, -9, -10, -12, -14 and -18. In the stria vascularis, claudin-1, -2, -3, -8, -9, -10, -12, -14 and -18 were expressed in the marginal cells, whereas the basal cells were positive only for claudin-11. In Reissner's membrane and the spiral limbus, the expression of claudin-1, -2, -3, -8, -9, -10, -12, -14 and -18 was detected. Furthermore, in the vestibule, claudin-1, -3, -9, -12, -14 and -18 were expressed in the sensory epithelia, whereas in the dark cell area claudin-1, -3, -8, -9, -12, -14 and -18 were detectable. These findings, i.e., very complex expression patterns of claudin species in the inner ear, would reflect the importance and the complexity of the barrier function of TJs in the inner ear.