The paper is divided into four sections. First, watershed management in Haiti is presented as a problem of voluntary collective action in which small watersheds are the common responsibility of a group of users. Second, this situation is given formal expression as a "public goods" problem, in which obligations to contribute time and labor to the maintenance and management of watersheds are treated as conditional or contingent commitments to cooperate (rather than defect). Third, an empirical analysis is presented in which key economic and cultural factors are tested to determine those that best explain the individual propensity to cooperate and the conditions necessary for collective action to emerge. Fourth, we interpret these results in light of the model, and suggest some generalizations and extensions of theoretical and empirical research on common property and collective action.