This article addresses the relationship between science and popular knowledge within Tanzania's Mafia Island Marine Park. It examines the ways in which various forms of knowledge circulate within the park, as well as how knowledge is evaluated and used by representatives of international organizations, national government officials, and Mafia residents. Despite the ostensibly 'participatory' goals of the marine park, island residents continue to be excluded because they fail to speak the language of the 'educated'. At the same time, forms of knowledge within the park serve as potent cultural markers of class status, ethnicity and 'modernity' in ways that buttress the social position of national and international elites and undermine that of island residents.