It is assumed that the partners in co-management negotiate their arrangements themselves and deal directly with each other. However, in Vietnam, fisheries co-management did not begin this way. This paper analyses the influence of international donor funded projects on the development and sustain ability of fisheries co-management systems in the Tam Giang-Cau Hai lagoon system, Vietnam. The findings indicate: (1) the development of co-management systems in these lagoons was led by donor funded projects; (2) the implementation approaches and exit strategies of the donors have affected the sustainability of the co-management systems; (3) the low participation of government in developing the co-management processes has resulted in poor cooperation between the projects and low integration of the projects' resources; and (4) the government has failed to develop "ownership" of co-management to become a true partner in co-management. Although technical and financial support from donors is essential for starting co-management systems in developing countries, this support does not necessarily ensure the ownership and sustainability of these systems. Policy and "know how" for developing and maintaining co-management systems are important for long term sustainability of co-management. This study also raises questions around the role of government in integrating and coordinating the resources of external agencies. (C) 2015 Elsevier Ltd. All rights reserved.