ABSTRACT. Brazil has a network of marine extractive reserves (RESEX), a form of marine protected area (MPA) using comanagement. The RESEX program aims to bring traditionally marginalized populations with natural resource dependent livelihoods into national development processes by empowering them to participate in governance and steward biodiversity conservation. We apply the socialecological systems framework (SESF) and collective action theory to diagnose challenges for comanagement in the Caete-Teperacu marine RESEX near Bragança, Brazil, a multiuse mangrove estuary supporting a small-scale crab fishery. We conducted key informant interviews and build on over 20 years of research in the region to provide an overarching analysis of the challenges facing comanagement. We describe the variables from the SESF in the case context and find that many social and ecological variables interact in clusters over time, and these clusters can be identified as themes, including (1) social and political momentum supporting the RESEX; (2) shifting perceptions of local residents and fishers; (3) patron-client relationships and social-ecological traps; (4) challenges with institutional fit; and (5) the interactions between harvesting closures, compensation, and dependence on local natural resources. Furthermore, we use collective action theory to help explain the role that each variable plays in either hindering or enabling successful governance. Our findings suggest that institutional resilience is needed to make RESEX adaptive to shifting social and political momenta. It could do this by providing more platforms for communication, deliberation, and knowledge exchange among the relevant actors. We believe our findings reflect broader challenges facing RESEX implementation throughout Brazil, and lessons can be learned for MPAs facing difficulties with the implementation of comanagement worldwide.