The saccus vasculosus of the ray, Dasyatis akajei (elasmobranch) was observed with the light and electron microscopes. The light microscopy revealed that the saccus vasculosus consists of follicles whose walls are lined with the epithelium made up of two cell types: the coronet cells with an apical cytoplasmic protrusion provided with many pin-like processes and the supporting cells distributed among the former. There are many blood capillaries and nerve bundles in the interstitial tissue among follicles. The general structures of the coronet cells, their Golgi apparatus, mitochondria, infranuclear homogeneous hyaline substance, PAS-positive granules and the intracellular glycogen were described. A special type of epithelial cell, flask-like in shape and filled with many mitochondria, were scattered in the saccus vasculosus. This cell type was recognized only by light microscopy, and failed to be observed under the electron microscope. The detailed ultrastructure of the coronet cell was observed by electron microscopy. The papillary cytoplasmic protrusion on the apical surface is provided with many pin-like processes whose tips are called globules and contain many vacuoles and vesicles. These pin-like processes are regarded as modified cilia. The homogeneous hyaline substance filling the infranuclear cytoplasm found by light microscopy is a peculiar cytoplasmic area composed of a thick layer or a network of fine filamentous bundles. Besides wide-spread smooth endoplasmic reticulum and filaments, electron microscopy revealed in the coronet cell cytoplasm also microtubules, vacuoles, vesicles, lysosomes, mitochondria and a Golgi apparatus. Small nerve bundles frequently penetrate into the epithelium and make a synapse-like structure adjacent to the coronet cell. Any evidence was found for secretory function neither in the coronet nor in the supporting cells. From the similarity in structure of the papillary protrusion with pin-like processes of the coronet cells to the olfactory rod with hairlets, the probable function of the saccus vasculosus was presumed to be sensory.