The ultrastructural development of utricular and saccular maculae in man and rat is described.In the human fetus with a crown-rump length (CRL) of 50 mm, the utricular and saccular epithelia consist of a pseudostratified layer of dark and light cells without nerve endings. In human fetuses of 110 and 220 mm CRL, as well as in 17− and 18-day-old rat embryos, it is already possible to identify sensory and supporting cells in various stages of differentiation. Nerve endings can also be found. Ciliogenesis is present in both sensory and supporting cells of the 17− and 18-day-old rat embryos.During epithelial differentiation a heavy calcium deposition occurs on ves-tibular cells of both man and rat. Otoconial mineralization increases gradually as epithelial maturation advances. Phagocytosis of otoconia by free macrophages occurs in 50-mm CRL human fetuses.Vestibular nerve myelination is asynchronous and is present in 110-mm CRL human fetuses and in 30-minute postpartum rats.