The paper summarizes findings from a survey of farmer-managed irrigation systems in the central Mexican state of Guanajuato. Three types of information are presented: structural characteristics (e.g., group size, inequality, reservoir capacity); system rules and organization; and performance indicators (maintenance and productivity). The average system comprises 123 households, each with an irrigated parcel of 3.3 hectares. A statistical model of rule-choice reveals that systems with greater land-holding inequality and older water users' associations are more likely to have chosen more complex water-allocation rules. An ordered-logit model of maintenance shows that greater social homogeneity is associated with higher maintenance levels.