This article addresses the questions of the extent to which, and the reasons why, western European trade unions may have privileged the protection of ‘insiders’ over that of ‘outsiders’. Temporary agency workers, among whom migrant workers are over-represented, are taken as a test case of ‘outsiders’. The findings from a comparison of Belgian and German multinational plants show that collective agreements have allowed a protection gap between permanent and agency workers to emerge in Germany, but not in Belgium. However, the weaker protection in Germany depends less on an explicit union choice for insiders than on the weakening of the institutional environment for union representation and collective bargaining. The conclusion suggests that European unions are increasingly trying to defend the outsiders, but meet institutional obstacles that vary by country.