The article argues that chronic overgrazing on the Navajo Reservation is the result of policies of the BIA and the Tribal Council. In support of that hypothesis the paper outlines the nature of property rights to grazing land on the Reservation. It shows that BIA and Tribal Council actions have led to the proliferation of small herds since 1933. Small herds increase the resource costs of establishing and enforcing property rights, and they increase the political costs of requiring compliance with grazing rules. Statistical tests reveal a close relationship between herd size and overstocking of the range. The paper concludes that in the absence of the constraints of a private property system, it may be necessary for an outside agency to regulate grazing to avoid the problem of the Commons.