Many cell functions, including maintenance of morphology, aggregation, motility, membrane ruffling, smooth-muscle contraction, cytokinesis in mammals and bud formation in yeast, are regulated through the dynamic reorganization of actin filaments. Although it has long been known that Ca2+ is a key regulator of the cytoskeleton, evidence is now accumulating that Rho, a Ras-related small GTP-binding protein, is another important regulator of these actin-dependent cell functions.