This article is interested with the legal mobilization of transnational interest groups at the European level (European Union and Council of Europe). It compares the legal and political lobbying strategies of two umbrella organizations - the European Women's Lobby (EWL) and the European Roma and Travellers Forum (ERTF), which seek respectively to promote the rights of women and those of Roma - focusing on their interactions with European institutions and law. The article analyses the contrasted relationship of these groups to legal mobil- ization as a rights advancement strategy, shedding new light on how law can be stra- tegically used by both strong and weak civil society actors. Beyond classical factors linked to organizational characteristics and identity, the differential usages of law by the two groups are explained by the role of strategic actors who adapt to the spe- cificities of the system of governance in the two policy sectors - gender equality and anti-discrimination.