The Wilms' tumor suppressor gene (WT1) is required for the formation of the mammalian metanephros, or adult kidney, and for the normal development of the mesonephros, the major mammalian embryonic kidney. In this report the isolation of a Xenopus gene closely related to the mammalian WT1 gene in both sequence and splicing pattern is described. Expression of this gene, xWT1, is restricted to the developing nephric system until late tadpole stages, which expression also begins to be observed in the heart. Within the nephric system, expression is observed in the dorsal portion of the splanchnic lateral plate in tailbud embryos, and in the glomus of early tadpoles. No expression is observed in the pronephric tubules or pronephric duct. The WT1 gene is therefore expressed in a similar temporal and spatial pattern in the vascularized portion of the amphibian pronephroi and in the mammalian metanephroi, arguing that it probably plays a similar crucial role in the morphogenesis of these very different kidney forms. The absence of expression in the developing pronephric tubules indicates that xWT1 is not required for the epithelialization of the tubular portion of the pronephros.