Punishment of shirkers is often an effective means of attenuating incentive problems and sustaining coordination in work teams. Explanations of the motivation to punish generally rely either on small group size or on a Folk theorem that requires coordinated punishment and, hence, highly accurate information concerning the behavior of each player. We provide a model of team production in which the punishment of shirkers depends on strong reciprocity: the willingness of some team members to contribute altruistically to a joint project and also to bear costs in order to discipline fellow members who do not contribute. This alternative does not require small group size, complex coordinated punishing activities, or implausible informational assumptions. An experimental public goods game provides evidence for the behavioral relevance of strong reciprocity and how it differs from unconditional altruism. (C) 2009 Elsevier B.V. All rights reserved.