Halfsmooth tongue sole(Cynoglossus semilaevis)belongs to Pleuronectiformes,Soleidae,Cynoglossus.It contains ZWZZ sex determination system and is a newly exploited and commercially important cultured marine flatfish in China.Females grow two to three times faster than males.Thus,the development of allfemale stock would be of significant benefit for aquaculture.To develop allfemale stock of C.semilaevis,the investigation of sex determination mechanism is necessary. A Dmrt3 cDNA fragment was cloned from C.semilaevis. The fragment was 228 bp long containing the DM conserved region.NJphylogenetic tree indicated that Dmrt3 gene of C.semilaevis had a high homology with that of Fugu rubripes, Oryzias latipes and Danio rerio. Real-time PCR showed that Dmrt3 was expressed in all seven tissues and extremely higher in male gonad(P<0.01).The expression of Dmrt3 gene in gonads of methyltestosterone treated male and neo-male was higher than that of normal males(P<0.01).But no significant difference was detected between normal female and the high temperature treated neo-male. This indicates that Dmrt3 gene is not indispensable for sex reversal.The high expression of Dmrt3 in MT treated fish is a result of methyltestosterone influence.From 43(days post hatching)to 5 months old, Dmrt3 was expressed very low.At 7 months old,the expression rose up.The highest expression of Dmrt3 occurred in ninemonthold gonad tissues. After 9 months old,the expression of Dmrt3 declined.According to morphological observation,we estimate that the ninemonth old testis is in phaseⅡ of fish gonad development stages.Large amounts of spermatogenic cells exist in phaseⅡ testis and the spermatogenic cells change to primary spermatocytes in phase Ⅲ.This implied that the expressions of Dmrt3 gene go up with the increasing of spermatogenic cells. Maybe Dmrt3 gene plays an important role in the formation of spermatogenic cells.Dmrt3 was considered not to be necessary for sex reversal.