This paper uses experimental data to test for a complementary relationship between formal regulations imposed on a community to conserve a local natural resource and nonbinding verbal agreements to do the same. Our experiments were conducted in the field in three regions of Colombia. Each group of five subjects played 10 rounds of an open access common pool resource game, and 10 additional rounds under one of five institutions— communication alone, two external regulations that differed by the level of enforcement, and communication combined with each of the two regulations. Our results suggest that the hypothesis of a complementary relationship between communication and external regulation is supported for some combinations of regions and regulations, but cannot be supported in general. We therefore conclude that the determination of whether formal regulations and informal communication are complementary must be made on a community-by-community basis.