Localization of calmodulin, a calcium binding protein, was identified in adult gerbil cochleas using paraffin section immunohistochemistry and immunogold electron microscopy with monoclonal antibody against bovine calmodulin. Immunoreactive calmodulin was abundant in inner hair cells (IHCs), outer hair cells (OHCs) and Boettcher cells of the cochleas. Other cell types containing calmodulin were marginal cells and basal cells of the stria vascularis, fibrocytes in the spiral ligament, spiral ganglion neurons and vascular smooth muscle cells. Immunogold labeling for calmodulin was observed in cuticular plate, stereocilia, and within cytoplasm of IHCs and OHCs. In OHCs the labeling was mostly observed in the region underlying lateral wall corresponding to subsurface cisterna. In IHCs the staining was diffuse in the cytoplasm and denser than that in OHCs. Boettcher cells showed dense staining along the microvillous projections facing to the intercellular spaces between Boettcher cells and Claudius cells and between the neighboring Boettcher cells. These distributions of calmodulin in the hair cells consist with the assumption that IHCs act as a true neurotransducer and OHCs as an active bi-directional mechanotransducer. The rich presence of calmodulin in Boettcher cells suggests that the cells may involve in mediating Ca(2+) regulation and play a distinctive active role in ion transport.