The nature and the submicroscopic structure of the tectorial and basilar membranes have been studied with phase-contrast, polarized-light, and electron microscopy; x-ray diffraction; and analytical chemistry. It is shown that the protein found in these membranes has nothing to do with collagen and elastic substance. Perhaps it may be classed in the same group as keratin, epidermin, myosin, and fibrinogen. The tectorial membrane consists of submicroscopic filaments which have a diameter of 96±4 Å, arranged fairly compactly, to form numerous transverse and a few longitudinal fibrous bundles. The basilar membrane consists of a supporting layer which is covered on the tympanic face by the basilar-membrane cells. The supporting layer is made of filaments with a diameter of 85±105 Å, strictly, arranged in a transverse direction. In the pars recta, they lie side by side, whereas, in the pars pectinata, they are grouped in variously sized fibers, separated by a cottony ground substance. These results are compared with those of Békésy's experiments, in order to find a relationship between structure and mechanical properties. The mechanical anisotropy observed in the tectorial membrane by Békésy finds a perfect agreement with the results of the submicroscopic investigations. The mechanical isotropy shown by Békésy in the basilar membrane apparently contrasts with the evident structural anisotropy. It could be explained by the lack of independence in the oriented structures and by the presence of the cottony ground substance which forms a system binding the filaments and the fibers. The transverse orientation of filaments and fibers is not the effect of mechanical forces alone, but could also be connected to other factors, such as those of morphogenesis and growth.