Electron microscopic observations were made of the cochlea in 5–6 month human fetus and 22–30-day rabbit fetus. The hair cells, supporting cells and afferent nerve fibers and endings can be identified among the rows of columnar cells that constitute the organ of Corti in 5–6 month human fetus. Intercellular fluid spaces and efferent nerve endings were not found in the human specimens studied here. The layer of cells next to endolymph in the stria vascularis, the future marginal cells, are separated from the cells beneath by a distinct basement membrane at the early stage of development. Later the basement membrane disappears as the mesodermal cells beneath begin to interdigitate with the marginal cells. A single kinocilium is present on the endolymphatic surface of the cells in the immature cochlea. These findings were correlated with the rabbit fetus cochlea and it is suggested that the last major event that the organ of Corti has, to begin to carry out its auditory function, efferent innervation, would occur in the later stages of human uterine