This article aims to examine the impact of group size on the provision of collective good provided by P2P file-sharing communities. Olson (1965) argued that small communities are more able to provide collective actions. Using an original database on Bittorrent file-sharing communities, our article finds a positive relation between the size of a community and the amount of collective good provided. However, the individual propensity to cooperate decreases with group size. These two features seem to indicate that P2P file-sharing communities provide a pure (non rival) public good. We also show that specialized communities are more efficient than general communities to encourage cooperative behavior. Finally, the rules designed by the managers of a community play an active role to stimulate voluntary contributions and improve the self-sustainability of file-sharing.Lire l’article complet