We describe the isolation and characterization of the RNA-binding protein XC3H-3b that is expressed during pronephros development. XC3H-3b is a member of the TTP/TIS family of CCCH tandem zinc-finger proteins, which are physiological stimulators of instability for the mRNA encoding tumor necrosis factor-alpha in certain cell types. XC3H-3b is localized primarily to the mesodermal tissues around the pronephros. Overexpression of XC3H-3b markedly and specifically inhibits kidney development. Morpholino-mediated knockdown of XC3H-3b also results in defects in nephrogenesis. In both cases, the expression of numerous pronephric marker genes, such as Xlim-1, Xpax-2, Xpax-8, Xwnt-4, and XWT1, is decreased and morphological development of the pronephric tubules is abrogated. We conclude that XC3H-3b plays an important role in the regulation of pronephros differentiation. This is the first report of a gene localized around the pronephros that regulates pronephros development.