Belgian municipalities rely heavily on surcharges on national income taxes as a source of income. In a context offederal government, such taxes create "fiscal externalities": decisions at one level of government affect the welfare at other levels. We focus on one such interdependency: EC-civil servants are not subject to national incame taxation. We show that this federal decision has far reaching budgetary effectsfor the municipalities (in the region of Brussels) where these people reside. Apart from its budgetary impact, this situation also baspolitical and social relevance.