PES are considered new incentive tools for managing both the environment and rural development in developing countries. However, designing mechanisms tailored to smallholder characteristics and rural development requirements remains a challenge, particularly in tropical countries where the forest sector is characterized by long-standing patterns of inequality and poverty. In order to switch from theoretical principles to a really innovative management tool addressing local issues, we argue that it is necessary to include as from the beginning the characteristics of agricultural dynamics, and to involve local stakeholders in co-designing the ES and in solving problems encountered in their delivery. Through an action-research framework, we explored the characteristics of a "pro-poor" watershed-based PES mechanism associated with a hydropower project. The study site is located in the agricultural frontier of a rainforest in eastern Madagascar. We built a local and inclusive knowledge system based on (1) local and scientific knowledge on the relationships between land use and water services, (2) stakeholder perceptions on the electrification project, and (3) the heterogeneity of livelihoods of targeted households. We were then able to clarify what the environmental service and governance scheme could be. The main results show the necessity of going beyond economic and hydrologic rationales that usually underlie watershed-based PES development. In conclusion, we point out the main elements that underlined the design of a pro-poor PES scheme able to accompany or promote the changes that are advisable for agriculture in the forest frontier.(Texte integral)