IUU (illegal, unreported, and unregulated) fishing causes severe detrimental social, economic, and environmental impacts worldwide. Although this practice is prohibited in Marine Protected Areas (MPA), practices such as fishing with compressor devices have been reported at the Marine Extractive Reserve of Corumbau (MERC), Brazil. In this case study we use data on MERC fisher knowledge and attitudes concerning compressor fishing gathered through semistructured interviews to discuss MPA strengths and weaknesses in preventing IUU fishing. Interviewees strongly oppose IUU fishing, complaining it is one of the main reasons for declines in local fishing yield. Respondents indicate that nonlocal compressor vessels were frequently observed prior to the establishment of MERC, and while such sightings are now less frequent, these vessels continue to appear. At the same time, respondents also indicate that marine environmental enforcement actions are rare. We discuss the roles, potentials, and limitations of MPAs in preventing illegal fishing and protecting fishers from competition with industrial fishing. We recommend the IUU be vigorously combated in Brazilian MPAs.