We evaluated the morphology and structure of the olfactory system in the Chinese black sleeper, (Lacepede) using histology. The olfactory system consisted of the olfactory sac, olfactory nerve, and olfactory bulb. The olfactory sac (the rosette) was fusiform in shape and located inside the olfactory chamber, which had two openings that allow water to flow through the rosette as the fish moves. There were 10–16 primary olfactory lamellae radiating from the wall of the olfactory chamber. These lamellae were longitudinally arranged and parallel to each other. The primary olfactory lamellae differed in their height and some possessed secondary olfactory lamellae. Olfactory lamellae were composed of the olfactory epithelium and central core. The olfactory epithelium consisted primarily of ciliated receptor cells, ciliated non-receptor cells, supporting cells, and basal cells. The axons of the primary olfactory receptor neurons in each rosette converged to form a pair olfactory nerves that exceeded 1 cm in length in a 17 cm fish. The paired olfactory nerves extended from the posterior ventral base of each rosette to the ipsilateral olfactory bulb. The two olfactory bulbs, in close contact with the telencephalon, were slightly oval and sessile. Each olfactory bulb consisted of three, roughly distinguishable layers, in order from the surface: (1) the olfactory nerve layer, containing the axons of the olfactory receptor neurons, (2) the glomerular and mitral cell layer, where the axons of the olfactory receptor neurons arborized into glomeruli and the secondary neurons (mitral cells) were scattered around glomeruli, and (3) the granule cell layer, consisting of densely-packed small size cells. Afferent fibers of nerve bundles reached the anterior bulb, spread along the periphery of the bulb and terminated on the dendrites of mitral cells in the glomerular and mitral cell layer. The olfactory nerve layer extended more caudally in the ventral lateral field than in the dorsal medial field. The glomerular and mitral cell layer was thinner in the medial field than in the lateral field of the olfactory bulbs. receptors were present in the olfactory system of the fish, with the largest number of PR occurring in the olfactory epithelium, followed by the olfactory bulb and the olfactory nerve. PR immunoreactivity was confined to the cell nucleus in the olfactory epithelium. We only observed a few neurons that were immunostained with antibodies against PR in the olfactory nerve in mature fish. In addition, we noted the presence of some PR immunoreactive cells on the vascular wall during the spawning season. We observed a higher density of PR immunoreactive cells in the olfactory bulb. The immunoreactive intensity in the bulb was higher in the lateral field and ventral fields than in the mid and dorsal fields, respectively. In the sections that were immunostained with antibodies against PR, we observed immunoreactive mitral cells in the glomerular and mitral cell layers. The numbers of were linked to the reproductive status of the fish.of immunoreactive cells in mature males and females was significantly higher than in immature males and females. Similarly, immunoreactive cells were not detected in the olfactory nerve in immature fish. Tn the was significantly higher in mature females than in immature females. To our knowledge, this is the first report documenting the distribution of.