The present paper explores the effect of the salience of collective consequences of opportunistic behavior in commons and anticommons dilemmas. Making this type of externalities salient was expected to increase the awareness of the conflict between collective and personal interests, especially in the anticommons dilemma. The results of a vignette study (Study 1, N = 100) and a laboratory experiment (Study 2, N = 55) confirmed our hypotheses, revealing more opportunistic behavior in the anticommons than in the commons dilemma when externalities were not made salient, while no significant dilemma effect was obtained when the externalities were made salient. Moreover, the results of Study 2 demonstrated that the dilemma effect on cooperation was mediated by externalities awareness. The positive effects of increments in externalities awareness on cooperation are discussed. Copyright © 2011 John Wiley & Sons, Ltd.