Light and electron microscopic immunolocalization of taurine, a sulfur-containing free amino acid, was investigated in the photoneuroendocrine pineal organ and the retinal photoreceptor and pigment epithelial layer of the ayu Plecoglossus altivelis, an anadromous fish. Intense immunostaining was found in the outer segments of pineal photoreceptors and retinal cone-like cells. Moderate but definite immunostaining was found in the cytoplasmic processes of pineal supporting cells, the outer segments of retinal rod-like cells, and the apical processes of pigment epithelial cells. Although the electron microscopic immunogold labeling was not completely coincident with the results of light microscopic immunostaining, concentrated immunogold particles appeared in the inner segments of photoreceptor cells, the cytoplasmic processes of pineal supporting cells, and the apical processes of pigment epithelial cells. These light and electron microscopic findings in taurine immunolocalization were discussed in relation to the functions of taurine known mainly from retinal physiology. It was suggested that the abundant taurine localization may be involved at least in the protection of photoreceptor outer segments against harmful factors, and in the transportation of nutrients and metabolites. The immunostaining for taurine is useful for the discrimination of different types of photoreceptor cells in the pineal organ and retina of fish.