The 2184 bp full-length brain P450 aromatase (termed P450aromB) cDNA encoding a protein of 498 amino acid residues was isolated from the brain of the half-smooth tongue-sole ( Cynoglossus semilaevis
) by using homologous cloning to clarify the role in sex differentiation. Sequence and phylogenetic analyses showed that the tongue sole P450aromB belonged to brain P450arom subfamilies. The putative P450aromB amino acid residues shared 48.3%-66.1% sequence affinity with other fish brain P450arom, and shared 34.2%-49.9% sequence affinity with ovarian P450arom of other fish. The RT-PCR analysis revealed that P450aromB mRNA was expressed at a high proportion in the brain and gills but less in the gonads and skin, but not in other tissues. However, the P450aromB decreased in the brain of sex-reversed males, treatment by Methyltestosterone or at high temperatures. These results suggested that the P450aromB is involved in the course of gonad development and sex determination in tongue soles.