Sustaining the regular flow of water from mountain forests is important for downstream stakeholders in seasonally dry tropical countries, and a watershed payment for ecosystem services (PES) scheme that links rural ecosystem service providers to urban water users through economic transfers may help to maintain water supply and forest habitat. A CARE/WWF project in the Uluguru Mountains of Tanzania has established a pilot watershed PES scheme. We trace the development of this scheme and outline its initial impacts. Memoranda of Understanding between companies in Dar es Salaam (the downstream water users) and farmers in the mountains provide the framework to deliver tangible financial benefits to local people, help change patterns of land use and potentially improve water quality. A number of lessons learned from this project are relevant for similar schemes elsewhere in the region. Overcoming the various challenges is essential for expanding the current pilot project to a scale where it delivers measurable changes in water quality for downstream users.