Transmission ratio distortion (TRD), the preferential inheritance of the t haplotype from t/+ males, is caused by the cooperative effect of four t-complex distorters (Tcd1-4) and the single t-complex responder (Tcr) on sperm motility. Here we show that Fgd2, encoding a Rho guanine nucleotide exchange factor, maps to the Tcd2 region. The t allele of Fgd2 is overexpressed in testis compared with wild type. A loss-of-function allele of Fgd2 generated by gene targeting reduces the transmission ratio of the t haplotype t(h49), directly demonstrating the role of Fgd2 as Distorter. Fgd2 identifies a second Rho G protein signaling pathway promoting TRD.