Résumé : Participatory conservation projects run by NGOs in developing countries imply involvement of communities in conservation efforts, to combine economic development with environmental preservation. We build an economic model explaining the emergence of participatory conservation and its contradictions linked to the conflicting incentives of local farmers, NGOs, and donors. The tragedy of the commons in a natural area justifies an NGO intervention. Contractual incompleteness calls for participatory conservation. However, if the revenue from the conservation project is uncertain, the community abstains from conservation unless the NGO allocates resources to agriculture. The NGO must deviate from its narrow mission to reach its broad mission. If the NGO is funded by conservation-oriented donors, it struggles to justify diverting resources to agriculture. Thus, the NGO faces a “size versus efficiency” dilemma: poorly conserving a larger area (non-cooperating local community, satisfied donors) or conserving well a smaller area (local community cooperation, unsatisfied donors).