In recent years, there has been a resurgence of civil disobedience over public land policy in the West, sometimes characterized by armed confrontations between ranchers and federal officials. This trend reflects renewed assertions that applicable positive law violates the natural rights (sometimes of purportedly divine origin) of ranchers and other land users, particularly under the prior appropriation doctrine and grounded in Lockean theories of property. At the same time, Native Americans and environmental activists have also relied on civil disobedience to assert natural rights to a healthy environment based on public trust, fundamental human rights, and other principles. This Article explores the legitimacy of natural law assertions that prior appropriation justifies private property rights in federal grazing resources. A subsequent article will evaluate the legitimacy of related assertions of natural law to support the public trust doctrine and other legal theories to support environmental protection.