Betaglycan is an accessory receptor for the transforming growth factor-beta (TGFbeta) superfamily, many members of which play key roles in kidney development. The purpose of this study was to define the role of this co-receptor on fetal murine kidney development. Stereological examination of embryonic and adult betaglycan heterozygous kidneys revealed augmented nephron number relative to littermate controls. Fetal heterozygous kidneys exhibited accelerated ureteric branching, which correlated with augmented nephron development at embryonic day (e) 15.5. In contrast, betaglycan null kidneys exhibited renal hypoplasia from e13.5 and reduced nephron number at e15.5. Quantitative real-time PCR analysis of e11.5-e14.5 kidneys demonstrated that heterozygous kidneys exhibited a transient decrease in Bmp4 expression at e11.5 and a subsequent cascade of changes in the gene regulatory network that governs metanephric development, including significant increases in Pax2, Eya1, Gdnf, Ret, Wnt4, and Wt1 expression. Conversely, gene expression in null kidneys was normal until e13.5, when significant reductions were detected in the expression of Bmp4 as well as other key metanephric regulatory genes. Tgfb1 and Tgfb2 mRNA expression was down-regulated in both nulls and heterozygotes at e13.5 and e14.5. The opposing morphological and molecular phenotypes in betaglycan heterozygote and null mutants demonstrate that the levels of betaglycan must be tightly regulated for optimal kidney development.