FilGAP is a newly recognized filamin A (FLNa)-binding RhoGTPase-activating protein. The GTPase-activating protein (GAP) activity of FilGAP is specific for Rac and FLNa binding targets FilGAP to sites of membrane protrusion, where it antagonizes Rac in vivo. Dominant-negative FilGAP constructs lacking GAP activity or knockdown of endogenous FilGAP by small interference RNA (siRNA) induce spontaneous lamellae formation and stimulate cell spreading on fibronectin. Knockdown of endogenous FilGAP abrogates ROCK-dependent suppression of lamellae. Conversely, forced expression of FilGAP induces numerous blebs around the cell periphery and a ROCK-specific inhibitor suppresses bleb formation. ROCK phosphorylates FilGAP, and this phosphorylation stimulates its RacGAP activity and is a requirement for FilGAP-mediated bleb formation. FilGAP is, therefore, a mediator of the well-established antagonism of Rac by RhoA that suppresses leading edge protrusion and promotes cell retraction to achieve cellular polarity.