The complete cDNA sequence of folliclestimulating hormone receptor(FSHR)gene was cloned by degenerate primer PCR amplification and RACE cDNA amplification for the first time.The length of the cDNA is 3105 bp,and it encodes a protein of about 704 amino acids,which contains the conserved seven transmembrane helix domains(TM-helix),and it belongs to glycoprotein hormone receptor(GHR)family.Phosphorylation site predictions identified 19 Ser,4 Thr and 5 Tyr potential phosphorylation sites in tongue sole,and only one protein kinase C phosphorylation site(⁴⁴¹T)was predicted.The Clustal X alignment also revealed the presence of specific signature sequences(e.g.⁸²CCAF,⁴⁸¹ERW,⁵⁹⁴FTD and ⁶⁶⁷NPFLY),which are highly conserved in GpHRs.And comparison with the halibut FSH-R also allowed the identification of 10 imperfect LRRs in the FSHR of Cynoglossus semilaevis.Tissue expression analysis showed that FSHR mRNA is expressed widely in C. semilaevis. Except the high levels in gonads,strong amplification signals were also detected in extragonadal tissues including the spleen,kidney and head kidney.Plasma 17β-Estradiol(E₂)seasonal changes are detected by radio immunoassay(RIA).The result shows that the lowest level appears in April and the highest peak of a year is in October.And we analyzed the relative expression level of FSHR in reproductive cycle of female.There is the highest level in October and the lowest level in January.