In order to elucidate the role of Sox9 in gonadal differentiation of Scatophagus argus, the full length cDNA of Sox9 was cloned from testis of S. argus using RT-PCR and rapid amplification of cDNA ends (RACE). To investigate effect of aromatase inhibitor letrozole (LE) on Sox9 gene expression and gonadal development, juveniles were fed with diets containing LE 50 mg/kg and gonadal development was observed histologically and the expression of Sox9 was measured with quantitative real-time PCR. RACE results (Fig. 1) showed that full length cDNA of 2 759 bp contained a 31 bp 5′-untranslated region (5′-UTR), a 1 288 bp 3′-untranslated region (3′-UTR) and a 1 440 bp open reading frame (ORF) encoding 479 amino acids. A HMG (high mobility group) box containing a specific motif, two nuclear localization signal (NLS) sequences and a nuclear export signal (NES), was found in the deduced amino acid sequence of SOX9 (Fig. 1). The amino acid sequence had a high similarity to SOX9 of other species, and the percent identity compared with Epinephelus akaara, Xenopus laevis, Homo sapiens, Gallus gallus, and Mus musculus was 61.5%-96.0%. The expression levels of Sox9 mRNA in different tissues were analyzed by quantitative real-time PCR. The highest expression level was detected in brain, intestine and fin (Fig. 3). Histological results showed sex reversal could be induced by LE, and the degenerating oocytes and proliferations of spermatogonia were observed in gonad from LE treatment S. argus (Fig.4). The expression level of Sox9 mRNA was up-regulated significantly at 20d, reached its peak at 40 d after treatment with LE but was declined markedly at 60 d (Fig.5) . These results suggest that Sox9 is highly conserved. Masculinization can be induced by LE treatment and Sox9 may be one of the important factors initiating the masculinization of S. argus during sex reserval.