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Cochlin, a product of the COCH gene, is a major constituent of the inner ear extracellular matrix. Type II collagen, a protein that contributes to structural stability, is also a component of this extracellular matrix. In this study, using the postembedding immunogold method, we demonstrate the localization of cochlin and type II collagen in the cochlear duct at the ultrastructural level. The immunolabeling of cochlin was observed in the fibrillar substance in the spiral limbus, beneath the inner sulcus cells, and in the basilar membrane, the spiral prominence and the spiral ligament. Immunolabeling of type II collagen was observed in the same fibrillar substance in the extracellular matrix of the cochlear duct. This localization of cochlin is consistent with the expected localization of type II collagen. The localization of cochlin and type II collagen indicates the important roles played by these proteins in the hearing process.