Full-length cDNA encoding common glycoprotein α subunit (CGα) was firstly cloned from half smooth tongue sole Cynoglossus semilaevis Günther by means of homology cloning and RACE PCR analyses. This sequence was submitted to GenBank with accession No. JQ364953. The whole cDNA length of tongue sole CGα gene was 685bp, including a 384bp open reading frame (ORF). The predicted mature tongue sole CGα consisted of 127 amino acids preceded by a signal peptide of 33 residues. The structure of mature CGα in tongue sole CGα was similar with those in the vertebrate CGα, including 10 conserved Cysteine residues and 2 N-linked glycosylation sites. Sequence comparison showed that tongue sole CGα mature peptide had higher identity to their orthologs of percomorphs (perciforms and pleuronectiforms; 60%~70% identity) than to cypriniformes (55%~60% identity). The CG(mRNA expression of tongue sole was detected in all the examined tissues by quantitative Real-Time PCR (qRT-PCR) method. The significant test analysis (P<0.05) for expression levels of tissues revealed that head kidney and spleen as well as spleen and liver did not differ significantly from each other. But other tissues showed significant different from each other. The highest expression level was in pituitary, followed by gill, kidney, muscle, ovary and brain; and lower level expression was found in other tissues including heart, head kidney, liver and spleen.