The fine structure of the odontoblasts in the sting rays, Dasyatis akajei, Dasyatidae, and Urolophus aurantiacus, Urolophidae, was examined using light and transmission electron microscopy. In the dentinogenesis stage, the odontoblasts have been classified into two types, that is, dark cells and light cells, based on differences in their fine structure. Many dark odontoblasts found along the predentine displayed well-developed organelles with secretory activity around the nuclei. They contained large amounts of expanded rER, widely distributed Golgi apparatus and secretory granules. In contrast, light odontoblasts showed a relatively clear cytoplasm and extended long processes which passed through the predentine and penetrated into the dentine. They contained large numbers of microtubules in the processes and many mitochondria around the nuclei. It is suggested that the light odontoblasts play an important part in material transport to the dentine and/or act as a sensory organ of the tooth. The dark odontoblasts seem to produce the organic matrix of the dentine and to prepare for mineralization in the dentine.