The structure and postnatal transformation of Kölliker's organ in the cat were studied by means of both light and electron microscopes. Kölliker's organ, which is located in the inner sulcus area of the cochlear duct during embryonic development of the kitten, appeared to undergo a major transformation during the first 30 days after birth. On the level of the organ as a whole the possible nature of this transformation is briefly discussed. On the cellular level, the transformation of this organ was found to involve a process of cellular autophagocytosis. Kölliker's organ consists of numerous tightly packed, tall columnar cells filling the inner sulcus. Autophagic vacuoles containing cell organelles were observed in these cells in all stages of transformation of the organ. Cellular autophagocytosis reduced the number of cells present in each section from approximately 50 in the newborn, to approximately 12 in the 30-day-old kitten. The apparent transformation of Kölliker's organ was observed as progressing from base to apex and from the limbus to the inner hair cell. The relationship of the tectorial membrane to Kölliker's organ and that of the tectorial membrane to the area in and around the inner hair cells are discussed.