IN THE mammalian cochlea the sensory cells are located to the organ of Corti where they form one row of inner and generally three rows of outer hair cells. They are arranged in a very regular geometrical mosaic and surrounded and interspaced by supporting cells, together forming the acoustic papilla. The organ of Corti has been extensively studied by light and electron microscopy. The vast majority of these publications have been based upon a study of sectioned, often decalcified material, and the status of the cochlea as a whole has been evaluated from reconstructions or studies of serial sections. Because of the inherent great difficulties very few studies have been made where every cell in the cochlea has been studied in relation to all adjacent cells. In reality only Retzius' in 1884 has made any real basic attempt to analyze the cytoarchitecture of the organ of Corti. In man studies