Otocysts explanted from 12th-gestation-day mice and maintained in organ culture under went a series of developmental changes which paralleled those that occurred in vivo and which resulted in the formation of a sensory epithelium of the vestibular type. At the time of explantation presumptive vestibular sensory epithelium consisted of cells that were undifferentiated, pseudostratified and rapidly proliferating. The only microtubules present were those of the mitotic apparatus. After 4 days of in vitro development cells comprising the presumptive vestibular sensory epithelium were less pseudostratified and more elongate; their nuclei had assumed a basal orientation and there was a clear maginal velum. Longitudinally oriented cytoplasmic microtubules were present at the apices of some cells; they were often grouped around a centriole which may have served as a nucleation centre for their assembly. After 7 days of in vitro development mitosis had ceased and supporting cells had innervated hair cells were present: both types of cells were always longer than they were broad and were often highly asymmetrical. Hair cells were flask- or columnar-shaped, with a nucleus situated in the basal third of the cell. Most mitochondria in hair cells were located in the apical third of the cell. The same distribution of mitochondria and nuclei was evident in supporting cells. Microtubules occurred throughout the length of the supporting cell and were always parallel to its longitudinal axis. In hair cells microtubules were more frequent than in supporting cells: the majority were parallel to the longitudinal axis of the cell but there were two exceptions. First, at the apex of hair cells some microtubules were oriented transversely and diagnonally: these were probably involved in the development and maintenance of the constricted apex of these cells. Secondly, microtubules appeared to be randomly arranged in the narrow region of the cytoplasm between the ventral surface of the nucleus and the base of the hair cells. Microfilaments were confined to the basal third of hair cells where their orientation paralleled that of microtubules. The possible functions of microtubules and microfilaments in the development of hair cells and supporting cells of the mouse vestibular epithelium are discussed.