Activated T cells rapidly assemble filamentous (F-) actin networks in response to ligation of the T cell receptor or upon interaction with adhesive stimuli in order to facilitate cell migration and the formation of the immune synapse. Branched filament assembly is crucial for this process and is dependent upon activation of the Arp2/3 complex by the actin nucleation-promoting factor Wiskott-Aldrich Syndrome protein (WASp). Genetic disruption of the WAS gene has been linked to hematopoietic malignancies and various cytopenias. Although the contributions of WASp and Arp2/3 to T cell responses are fairly well characterized, the role of the mammalian Diaphanous (mDia)-related formins, which both nucleate and processively elongate non-branched F-actin, has not been demonstrated. Here, we report the effects on T cell development and function following the knock out of the murine Drf1 gene encoding the canonical formin p140mDia1. Drf1(-/-) mice develop lymphopenia characterized by diminished T cell populations in lymphoid tissues. Consistent with a role for p140mDia1 in the regulation of the actin cytoskeleton, isolated Drf1(-/-) splenic T cells adhered poorly to extracellular matrix proteins and migration in response to chemotactic stimuli was completely abrogated. Both integrin and chemokine receptor expression was unaffected by Drf1(-/-) targeting. In response to proliferative stimuli, both thymic and splenic Drf1(-/-) T cells failed to proliferate; ERK1/2 activation was also diminished in activated Drf1(-/-) T cells. These data suggest a central role for p140mDia1 in vivo in dynamic cytoskeletal remodeling events driving normal T cell responses.