Based upon the definition of security community and its application to the Common Market of the South, Mercosur, the author examines the subregion's capacity for responding to current threats to its security, emerging primarily from dynamics that transcend the state and musí thus be counteracted by collective efforts involving both the state and civil society.The article explores the historical record of Mercosur with the goal of illustrating that integration processes are not exempt from security problems; in fact, the subregión is threatened by arms proliferation and common violence. As a result, civil society has adopted a diverse series of actions that constitute an important example of collective action and that,as in the case of the threats mentioned, may become transnationalized, successfully reproducing themselves in other parts of the región.