Phase contrast and electron microscopy was used to study the maturation of the organ of Corti in mice. The hair cells, supporting cells, and afferent nerves and nerve endings can be identified at birth among the rows of columnar cells that constitute the mouse organ of Corti at birth. Intercellular fluid spaces develop by the 6th day as these cells separate to progress towards the architecture of an adult organ of Corti. Not until the 10th day after birth can efferent nerve endings be identified. Correlating our findings with the studies of the development of auditory function reported by Alford & Ruben (1963), we have concluded that all major structures are well-developed before auditory responses can be elicited. Efferent innervation is the last major event before the organ of Corti begins to function.