We provide experimental evidence on the effects of non-monetary punishment by peers among communities of Uruguayan fishers exploiting a common pool resource (CPR). We combined this treatment with an in-group (groups from a single community) / mixed group (groups composed of fishers from different communities) treatment. Our aim is to compare the effects of non-monetary sanctions in a context in which individuals exploiting a CPR belong to different communities relative to the case in which only individuals from the same community are allowed to exploit the resource. We find that mixed groups—unlike in-groups—reduce their exploitation of the resource in response to the threat of punishment. We do not find any differences in behavior between in-groups and mixed groups when the possibility of being punished is not available. The effectiveness of non-monetary punishment is reduced because cooperation was not perceived as the unique social norm. In such cases there is substantial antisocial punishment, which leads to increased extraction of the CPR by those who are unfairly punished. These findings indicate that effective peer punishment requires coordination to prevent antisocial targeting and to clarify the social signal conveyed by punishment.