The livelihoods and food security of many Cambodians depend heavily on inland capture fisheries, so the sustainable management of these fisheries is very important. Notwithstanding, the sustainability of Mekong fisheries is threatened by increasing fishing pressure and habitat modifications. Current management is considered insufficiently capable of controlling levels of exploitation and achieving equitable distribution of the resource, and the Cambodian government is promoting co-management initiatives as a way of addressing these issues. Using an institutional analysis approach, the current performance of co-management in a Mekong mainstream fishery was assessed, and factors limiting success identified. Results suggest that performance and sustainability of co-management are currently constrained by a lack of clearly defined property rights and resource boundaries, a mismatch between resource scale and management initiatives, and an absence of enabling legislation. Furthermore, objectives for management differ between stakeholders and are not well defined overall. While enabling legislation due to come into effect shortly is expected to improve performance and sustainability of co-management, boundary and scale issues will continue to pose challenges to co-management in open systems such as the Mekong mainstream. As a result, calls for innovative solutions, such as a federation of co-management schemes, will continue. In addition, increasing dependence on fishing, and external threats to resources need to be addressed if sustainability is to be improved