Immunocytochemical and immunoelectron microscopic localizations of a salmonid olfactory system-specific protein (N24) were investigated in the olfactory system (the olfactory epithelium, the olfactory nerve and the olfactory bulb) of kokanee salmon (Oncorhynchus nerka) by using a specific antiserum to N24. N24 immunoreactivities were observed in the cytoplasm of ciliated and microvillous olfactory receptor cells but were not observed in the supporting and the basal cells in the olfactory epithelium. Gold particles showing immunoreactivities for N24 were scattered in the cytoplasm of the dendrites of olfactory receptor cells. Some particles were concentrated on vesicular structures, but none were observed in the membrane of olfactory receptor cells. N24 immunoreactive axons were terminated at the glomerular layer near the mitral cells in the olfactory bulb. In an olfactory rosettectomy experiment, N24 immunoreactivity in the olfactory bulb vanished fifteenth day after the excision of the olfactory rosette. These results reveal that the olfactory receptor cells produce N24 which exists in both dendrites and axons of the olfactory receptor cells, and suggest that N24 may participate in neuromodulation in the olfactory system of kokanee salmon.