Recently, in this journal, René Millon and others discussed the results of their study of irrigation agriculture in the contemporary social setting of the Teotihuacan Valley, Central Mexico. In introducing their study, the authors briefly noted comparable investigations (including my own) and came to the general conclusion that “any system of irrigation agriculture creates its own distinctive potential for both cohesion and conflict, whatever may be the social system of the people who practice it.” Do social systems play as passive a role in determining the social effects of irrigation agriculture as the statement implies? I wish to suggest a distinction which may be important in seeking answers to this question and then to briefly compare certain aspects of Millon's Mexican example with my own study of irrigation agriculture and social organization in the Southern Euphrates Valley of Iraq.