In the late nineteenth century, with the expansion of their harbours and the growth of transatlantic mobility, the port cities of Antwerp and Rotterdam became home to economically important and large migrant communities. In a context marked by the often-claimed rise of the nation state, when national legislation concerning nationality and citizenship was shifting, local authorities and citizens played an important but still underestimated role when it came to enforcing the naturalisation of foreign nationals. Applications for naturalisation in both Antwerp and Rotterdam were firmly rooted in the local context, and economic performance was key to the police commissar’s support of an applicant’s case towards the national authorities. By comparatively analysing individual applications for naturalisation in Antwerp and Rotterdam, this paper argues that the close relationship between the nation-state and the mechanisms of legal inclusion and exclusion on which it rested, has to be relativised.