Receptor cells in the ear are excited through the bending of sensory hairs which project in a bundle from their surface. The individual stereocilia of a bundle contain filaments about 5 nm in diameter. The identity of these filaments has been investigated in the crista ampullaris of the frog and guinea pig by a technique of decoration with subfragment-1 of myosin (S-1). After demembranation with Triton X-100 and incubation with S-1, "arrowhead" formation was observed along the filaments of the stereocilia and their rootlets and also along filaments in the cuticular plate inside the receptor cell. The distance between attached S-1 was 35 nm and arrowheads pointed in towards the cell soma. It is concluded that the filaments of stereocilia are composed of actin.