This work presents the results of framed field experiments designed to study the co-enforcement of access to common pool resources. The experiments were conducted in the field with artisanal fishers in Chile. In the experiments, members of a CPR group (called insiders) not only decided how much to harvest but also invest in monitoring to deter poaching by outsiders. Sanctions for poaching were exogenous as if provided by a government authority. Treatments varied the level of the sanction and whether the monitoring investment was voluntarily provided by insiders or collectively provided by a group vote. Results suggest that co-enforcement can reduce poaching by outsiders and increase harvests by insiders, but the insiders’ monitoring investments tended to offset their gains in harvest earnings. Higher poaching sanctions and voting on monitoring contributions, separately and together, led insiders to invest enough in monitoring to eliminate the average gains from poaching. However, poaching was not eliminated despite zero or negative average poaching earnings.