A description is given of the general architecture of the sensory epithelia of the labyrinth of the elasmobranch fish Raja clavata, including an electron-microscopic study of the ultrastructural organization of the sensory hair cells and their hair processes, as well as of the supporting cells. The innervation of the hair cells and the associated synaptic structures are also described. The hair cells are found to be of uniform shape and cytological appearance. However, there exists a pronounced dimorphism so far as the sensory hair processes are concerned. These consist in each cell of a varying number of stereocilia with which a single kinocilium is associated. This has the typical structure of a cilium with 9+2 longitudinal filaments. Two widely different diameters of stereocilium are encountered in neighbouring hair cells, and it is suggested that this may point to a fundamental dimorphism among the sensory cells. The topographic arrangement of the kinocilia appears to be of functional significance and it is postulated that their position within each hair bundle is related to the direction of excitatory and inhibitory displacement of the hair bundle in the course of mechanical stimulation of the sensory cell. This situation is explored and analyzed for all sensory epithelia of the labyrinth, and the findings are related to what is known about their mode of function. The root structures of the kinocilia are described and it is shown that they, too, appear to be functionally polarized. Each supporting cell bears a single rod-like hair process which resembles the kinocilia in so far as it has at least 9 longitudinal filaments, but differs from it in certain aspects of its root structure. The question of the functional significance of this ciliary rod is discussed. Three types of nerve endings are found. They differ in the size and cytoplasmic contents of their terminal structure and are described as non-granular, granular, and small electrondense nerve endings. Whether the third type resembles the ending of an independent nerve supply cannot be decided.