How do social movements mobilize diverse groups to protest the invisible threats of new technologies like genetically modified seeds? The controversy over genetically modified maize in Mexico provides an opportunity to reconstruct theories about science, democracy, and transnational activism in a previously unexplored context-an international scientific controversy centrally involving a Latin American country. Through qualitative interviews with activists and their opponents, participant observation of meetings, protest activities, and maize festivals, and analysis of a variety of documentary sources, I will analyze the formation of a transnational advocacy network around the issue of Mexican maize, identify the strategies used by this network to exert pressure both on the state and on the scientific community, and examine the challenges to these efforts. This study engages four major research questions: 1) How do activists evaluate the claims of scientists and develop beliefs about the presence and effects of GM maize in Mexico? 2) Under what circumstances do activists participate in or transform processes of knowledge-production about the presence and effects of GM maize in Mexico, and how do their opponents attempt to exclude them from these processes? 3) Do social relations within the transnational activist network reproduce global power relations and a conventional lay-expert divide, or does the network challenge those power relations? 4) How do scientific controversies, schisms within the scientific community, and new discoveries affect the mobilization of the transnational activist network?