The high sensitivity of human hearing is believed to be achieved by cochlear amplification. The basis of this amplification is thought to be the motility of mammalian outer hair cells (OHCs), i.e., OHCs elongate and contract in response to acoustical stimulation. This motility is made possible by both the cytoskeleton beneath the OHC plasma membrane and the motor protein prestin distributed throughout the plasma membrane. However, these factors have not yet been fully clarified. In the present study, therefore, attempts were made to observe the ultrastructure of the cytoskeleton of Guinea pig OHCs and to identify the motor protein prestin expressed in the plasma membrane of Chinese hamster ovary (CHO) cells by atomic force microscopy (AFM). Results indicate that the OHC cytoskeleton is comprised of circumferential actin filaments and spectrin cross-links and that particle-like structures with a diameter of 8–12 nm which exist in the plasma membrane of the prestin-expressing CHO cells are most likely be prestin.