The fragility of migrants is closely linked to citizenship status and to questions of vulnerability. Turner notes that one result of genuine human vulnerability is the creation of social institutions “that come to constitute what we call ‘society’“ (Turner, 2006). However, as debates on illegal migration show, access to social institutions is in many ways limited by citizenship status. On the other hand, developments in recent years have shown how non-citizens can also transform the rules of citizenship and gain access to rights (e.g. Isin, 2009), thereby shaping rights and responsibilities, modes of participation and belonging as key dimensions of the citizenship regime (Jenson & Papillon, 2003). This article takes the link between migrant fragility and human vulnerability as a starting point for elaborating the fragility of migration from a citizenship perspective. Sanctuary cities in the United States provide an example of how citizenship is negotiated and transformed at the local level and the contradictions implicit in these processes. Focusing on San Francisco and the ways in which immigrants have gained limited opportunities due to sanctuary legislation, the article shows that the fragility of migration is not only linked to citizenship, but is also a field of tension and a process that needs to be addressed as such.