Abstract: Olfaction is an important tool for fish to perceive the external environment. To study the role of olfactory receptors in the spawning migration of Coilia nasus, the full-length sequence of olfactory receptor gene MOR-51I2 was cloned from the migratory C. nasus using genome walking technology. The MOR-51I2 gene was composed of a single exon with the open reading frame of 999 bp. The 3' untranslated region of this gene had a microsatellite sequence formed by (AC) n inserted by several T and G. In addition, the microsatellite sequences in different ecotype of C. nasus had significant length difference. The MOR-51I2 was a G-protein-coupled receptor with seven hydrophobic alphahe-lical transmembrane structures. The MOR-51I2 protein shared homology (>51%) with known related olfactory recep-tors from other fishes and it reached 83% homology with the olfactory receptor 51I2-like protein in Clupea harengus. The MOR-51I2 gene was highly expressed in the female olfactory rosettes of the settlement population of C. nasus, which is two times compared with that in male olfactory rosettes and 80 to 100 times compared with that in testis and ovary. The MOR-51I2 gene was expressed weakly in liver and gills and was almost not detected in the muscle, heart and eye. The expression level of female olfactory rosettes was 5 times higher than that in male olfactory rosettes in the migratory population. The expression level in female olfactory rosettes of the settlement population was 5 times higher than that in female olfactory rosettes in the migratory population, while the expression level in the male olfactory rosettes of the migratory population was 2 times higher than that in male olfactory rosettes in the settlement population. These results suggest that MOR-51I2 gene may not only regulate the olfactory function of C. nasus, but also mediate gonad development, spawning migration, and ecological differentiation of C. nasus.