This article examines the gap between the unionization rate of local and migrant workers in 14 Western European countries. The analysis reveals that the lower unionization rate of migrant workers can be attributed only in part to the impact of labour market segregation. Moreover, the gap between the unionization rate of local and migrant workers varies substantially across countries. We find that this gap is larger in those countries in which unions enjoy organizational security either in the form of state financing or a single dominant confederation.