Abstract. This article focuses on the political claims made by immigrants and ethnic minorities in France and Switzerland. We look at cross-national variations in the overall presence of immigrants and ethnic minorities in the national public space, and the forms and content of their claims. Following a political opportunity approach, we argue that claim-making is affected both by institutional opportunities and by national models of citizenship. The civicassimilationist conception of citizenship in France gives migrants greater legitimacy to intervene in the national public space. Furthermore, the inclusive definition of 'membership in the national community' favors claims pertaining to minority integration politics. However, the pressure toward assimilation to the republican norms and values tends to provoke claims for the recognition of ethnic and cultural difference. Finally, closed institutional opportunities push migrants' mobilization to become more radical, but at the same time the more inclusive model of citizenship favors a moderate action repertoire of migrants. Conversely, the ethnic-assimilationist view in Switzerland leads migrants to stress homeland-related claims. When they do address the policy field of ethnic relations, immigration and citizenship, they focus on issues pertaining to the entry and stay in the host society. Finally, the forms of action are more moderate due to the more open institutional context, but at the same time the action repertoire of migrants is moderated by the more exclusive model of citizenship. Our article is an attempt to specify the concept of 'political opportunity structure', and to combine institutional and cultural factors in explaining claim-making by immigrants and ethnic minorities. We confront our arguments with data from a comparative project on the mobilization on ethnic relations, citizenship and immigration.A great deal of work on contentious politics during the past three decades has followed the political process approach in trying to account for the emergence, dynamics and outcomes of social movements, both comparatively across countries and over time (e.g., McAdam 1999;McAdam et al. 2001;Tarrow 1998;Tilly 1978). The central concept in this perspective is that of political opportunity structures (Brockett 1991;Della Porta 1995;Kitschelt 1986;Kriesi et al. 1995;McAdam 1996;Tarrow 1998), which refers to political-institutional aspects of the movements' context such as the relative openness or closure of the institutionalized political system, the stability or instability of that broad set of elite alignments that typically undergird a polity, the presence or absence of elite allies, and the state's capacity and propensity for repression (McAdam 1996).