Receptor cells in the ear are mechanically excited through displacement of sensory hairs, stereocilia, in relation to a sub-surface platform, the cuticular plate, into which rootlets of the stereocilia insert. The presence of actin in inner ear sensory organs and receptor cells was established by gel electrophoresis, by labelling with antibodies against actin, and by electron microscopy after decoration with subfragment-1 of myosin. The latter method was used to determine the functional orientation of actin filaments found to be present in the mechanosensitive region of the receptor cells. Actin filaments were demonstrated in the stereocilia and their rootlets, in the cuticular plate and in relation to the zonula adherens surrounding the top of the cell. Filaments which run parallel to the cell surface were found in the cuticular plate and zonula adherens. Some filaments associated with the zonula adherens had a functional orientation opposite to that of more centrally located filaments in the cuticular plate. A structural complex consisting of a solid filament surrounded by actin filaments in hexagonal packing was found in the periphery of the cuticular plate. The possibility is suggested that the central filament is myosin.