The molecular mechanisms of vertebrate ZZ/ZW-type sex-determining systems remain unclear. We recently indicated that a W-linked gene, DM-W is a likely ovary-determining gene in Xenopus laevis. We first examined whether Cyp19 for estrogen-synthesizing enzyme P450 aromatase and Foxl2 showed female-specific expression in developing gonads. Both genes showed much higher expression in ZW than in ZZ gonads during and after sex determination. Importantly, transgenic ZZ gonads expressing exogenous DM-W at the sex-determining stage showed a ZW-type pattern of Cyp19 and Foxl2 expression. These results suggest that DM-W up-regulates Cyp19 and Foxl2 expression to guide primary ovary development in X. laevis.