Studies of the electoral behaviour of immigrants in Western Europe and North America have revealed a remarkably coherent cross-national voting pattern. Immigrants from the non-Western world hold a strong preference for left-of-centre parties. This unusual expression of group voting is so stable over time that it has been referred to as an ‘iron law’. There is, however, a dearth of scholarly research on this phenomenon. This article tests two explanations for the left-of-centre preferences of immigrants in Norway. The first is that the ideological and socio-economic composition of the immigrant electorate explains the preference for left-of-centre parties. If so, these voters' ethnic or immigrant background is not in itself decisive on Election Day. The second hypothesis is that immigrant voters engage in group voting, in which one's ethnic or immigrant background is significant and trumps other concerns when voting. This would express itself in a coherent voting pattern that cannot be explained by other factors. We also expect those who engage in group voting to favour candidates with similar ethnic backgrounds as themselves. The group voting hypothesis finds the strongest support. The immigrant vote appears to be driven by group adherence, rather than by ideology or social background.