The existing literature provides different accounts on the strategies of unions regarding marginal workers. It has been argued that under increasing labour market segmentation, unions have either to prioritize their core constituencies and to seek compromises with management, or to adopt inclusive strategies towards peripheral workers to counterbalance eroding bargaining power. This article shows that both strategies represent equally viable options to protect the interests of unions' core members. The strategic choice depends on the (perceived) competition between core and peripheral employees related to employers' personnel strategies; this affects the possible alignment of interests between unions' core members, on the one hand, and either management or peripheral employees, on the other. Our historical analysis of union strategies towards agency workers in the German metal sector illustrates this mechanism, and identifies institutional change towards liberalization as the trigger for aggressive segmentation strategies by employers and for inclusive union strategies.