beta-Arrestins were initially shown, in conjunction with G protein-coupled receptor kinases, to be involved in the desensitization and internalization of activated seven-transmembrane receptors. Recently, beta-arrestin 2 has been shown to act as a signal mediator in mitogen-activated protein kinase cascades and to play a positive regulatory role in chemotaxis. We now show that beta-arrestin 1 is required to activate the small GTPase RhoA leading to the re-organization of stress fibers following the activation of the angiotensin II type 1A receptor. This angiotensin II type 1A receptor-directed RhoA activation and stress fiber formation also require the activation of the heterotrimeric G protein G(alphaq/11). Whereas neither beta-arrestin 1 nor G(alphaq/11) activation alone is sufficient to robustly activate RhoA, the concurrent recruitment of beta-arrestin 1 and activation of G(alphaq/11) leads to full activation of RhoA and to the subsequent formation of stress fibers.