Prior research shows that third-party agents are necessary to promote cooperation when groups are large and spatially diffuse. I explore whether this proposition holds in the self-governing sport of Ultimate. While the size of the community and spatial diffusion of the sport theoretically suggests limited decentralized control, the widespread implementation of a refereed system has not yet emerged. Instead, I find with qualitative methods that cooperation in Ultimate is the result of the sport being federally controlled and embedded within tiers of organizational constraint that promote informal regulation of competitions through norms, reputations, and self-discipline.