The Wilms' tumor suppressor gene, WT1, plays a crucial role during early urogenital development in mammals. To elucidate the function of WT1 in other vertebrates, we isolated the Xenopus WT1 homolog (XeWT1) from a testis cDNA library. Comparison of the XeWT1 protein with other WT1 proteins revealed that the zinc finger domain of XeWT1 is identical to that of the human WT1 except for the first two amino acid residues. An alternative splice II site, located between the third and the fourth zinc finger is also conserved. In the transcriptional regulatory region, however, three domains, a glycine stretch, a proline stretch and one alternative splice site which are present in the mammalian WT1, are not conserved in amphibians. The XeWT1 gene is expressed in testis and kidney, and whole mount in situ hybridization revealed that the onset of the WT1 gene expression coincides with the early stages of pronephros development in Xenopus. These findings implicate the involvement of WT1 protein during urogenital development in amphibians as well as in mammals.