Gem is a Ras-related protein whose expression is induced in several cell types upon activation by extracellular stimuli. With the aim of isolating the cellular partners of Gem that mediate its biological activity we performed a yeast two-hybrid screen and identified a novel protein of 970 amino acids, Gmip, that interacts with Gem through its N-terminal half, and presents a cysteine-rich domain followed by a Rho GTPase-activating protein (RhoGAP) domain in its C-terminal half. The RhoGAP domain of Gmip stimulates in vitro the GTPase activity of RhoA, but is inactive towards other Rho family proteins such as Rac1 and Cdc42; it is also specific for RhoA in vivo . The same is true for the full-length protein, which is furthermore able to down-regulate RhoA-dependent stress fibres in Ref-52 rat fibroblasts. These findings suggest that the signalling pathways controlled by two proteins of the Ras superfamily, RhoA and Gem, are linked via the action of the RhoGAP protein Gmip (Gem-interacting protein).