This study examined attitudes towards weak and strong assimilation in a sample of 3,121 pupils from 70 secondary schools in Brussels with a multilevel linear analysis. Controlling for socio-demographic variables, members of some ethnic minority groups endorsed weak assimilation to a greater extent than the majority. Within members of the majority, the French-speaking Belgians held similar attitudes towards assimilation to the Dutch-speaking Belgians. Among ethnic minorities, the identification of pupils from some foreign-origin countries with the mainstream society was positively associated with the endorsement of strong assimilation, while their ethnic identification remained non-significant. Intergroup friendship among pupils of foreign origin was positively related to strong assimilation. Moreover, pupils of foreign origin who experienced institutional discrimination against ethnic minorities in favour of the majority endorsed strong assimilationism to a lesser extent. Among the majority, the perception that people of foreign origin are victims of discrimination was positively related to the endorsement of weak assimilation. However, this association was mediated by the experience by the majority of institutional discrimination against the majority and in favour of ethnic minorities. In-group identification among the majority was not associated with the endorsement of assimilation. The results and their implications for future research and policy makers are discussed. © The Author 2010. Published by Oxford University Press. All rights reserved.