Using a comparative approach to the public debate on GMOs in France and the United Kingdom, this paper analyzes the interactions between community actions and participative mechanisms. These mechanisms (consensus conferences, citizen juries, interactive technological evaluation, etc.) are nurtured by the crucial work of the groups involved in raising awareness for a public issue and developing alternatives to current dominant opinions. Thus action and participative mechanisms have the potential to offer a complementary method of social control over the direction taken by scientific and technical development, as well as to enlarge the frameworks for risk analysis. We analyze here the reasons for the problematic nature of appropriation of what is produced in the "small world" of participatory mechanisms by community actors and by the advocates of traditional decision-making procedures. Finally, we discuss the possibilities for moving beyond such obstacles.