The estimation of production function for groups of rice farmers with and without an irrigators' association (IA) reveals that the availability of irrigation water is a critical determinant of the technical efficiency of individual farmers. It is found that the mean level of technical efficiency of farmers located in the irrigation system with an IA is higher than that of farmers located in the system without an IA. Likewise, the variation of technical efficiency of farmers located in the system with an IA is lower than without an IA. These results suggest the critical role played by an IA in the O&M of irrigation systems and in ensuring the efficient and equitable distribution of water among farmers. Efficient and equitable distribution of scarce water in irrigation systems might be enhanced if IA implemented rational water rotation as well as better maintenance activities. Our study supports the hypothesis that the existence of an IA does affect the productivity of rice farming positively. Certainly, there could be many factors that are not included in this study that possibly affect the technical efficiency of farmers. In this sense, our evidence is circumstantial. Further study is necessary to single out the impact of an IA on the technical efficiency of farmers.