Sex is determined by various transcription factors in vertebrates. SRY on the Y chromosome in mammals has been isolated as the sex-determining gene. In birds, Dmrt1 on the Z chromosome of chickens is expressed in the indifferent gonad before sex determination. In amphibians, the sex-determining gene is not known yet. To examine whether Dmrt1 is expressed during sex determination in Xenopus laevis, we first isolated the cDNA, and examined its expression in gonadal development by RT-PCR analysis. The expression of Xenopus Dmrt1 (xDmrt1) was observed exclusively in the testis amosis. The expression was much higher in the testis than in the ovary. This was a pattern different from that in human, mouse, and fish such as tilapia and rainbow trout. In addition, the Dmrt1 mRNA was first detected in the gonad/mesonephros complex of tadpoles at stage 53, at which time the gonads were still indifferent. Taken together, the results suggest that Dmrt1 may be involved in gonadal differentiation of Xenopus.