The avian tectorial membrane is a thick massive-appearing structure permeated by cavities, which probably facilitate the diffusion of endolymph. As revealed by scanning and transmission electron microscopy, the cavities are arranged in a characteristic honeycomb-like pattern and each hair bundle is enclosed in an alveolus. The open ends of the alveoli show the impressions of sensory hairs on one side. The rims about the cavities are attached to the microvilli of the supporting cells by means of fibrous material. These morphological aspects are compared with those reptiles and mammals, and the functional significance of the fibrillar anchors is discussed.