This paper presents the results of an investigation into the avian organ of Corti. Its form and surface area have been estimated and the total number of sensory cells calculated. As described by other authors, the sensory cells occupying the apical and anterior parts of the basilar papilla are tall and slender while those occupying the basal and posterior parts are short and flat. The more exact distribution of these two kinds of cells has been mapped. The morphological polarization of the organ is directed away from the attachment of the tectorial membrane towards the cartilage posterior (ligamentum spirale), as is the case in mammals.