The past decade has seen a revival of the long-disfavored concept of frontier, which now elucidates the interdependence between political centers and their naturally rich peripheries in the Global South. "Frontier" has referred to the cyclical discovery of and competition over new resources, and the transformation of power relations in a given space. We contribute to this discussion by exploring its relation to nature conservation. Examining conservation practices along Mexico's southern border, this article addresses conservation as an unsettled process that pierces the frontier's politics of nationalizing space and communities. In this way, conservation practices become negotiations over ownerships of nature. These negotiations involve resistance, adaptation, and counter-conservation, and they also influence human-nature relationships. The article shows that conservation practices relate to spheres of both the state and local communities in ways that allow the study of the frontier beyond given binaries and oppositional forces and toward contested, multiscale deliberations about the control of, access to, rights, and attachments to nature.