Why are populist radical right parties (PRRPs) more successful in some countries than in others? This question is analysed here by focusing on Belgium. While Flanders (the northern, Dutch-speaking part of Belgium) was home to one of the strongest far-right movements in Europe, Wallonia (the southern, francophone part) has remained ‘immune’ to such tendencies. The article argues that different historical experiences have given rise to a hostile political environment for PRRPs in Wallonia, where mainstream parties and the media have created a successful cordon sanitaire. In Flanders, mainstream parties and the media have gradually become more accommodative towards PRRPs. By emphasizing the sociopolitical context in which parties operate, the findings suggest that the reactions of mainstream parties and the media are crucial to understanding the success of PRRPs. The conclusion reflects on potential lessons to be drawn from the Belgian case for mainstream parties and media practitioners elsewhere.