Existing scholarship claims that national policy has been much more deeply and dramatically Europeanized by the European Union (EU) than the administrative structures of national government. This finding appears to confirm the explanatory power of 'new' institutional theories, which emphasize the resilience of institutions in the face of strong pressures promoting greater convergence among dissimilar European political systems. Of the various national government structures that could be Europeanized, departments (or ministries) are especially important because they are primarily responsible for reconciling (or 'fitting') the demands of EU and national policy. However, existing scholarship does not fully investigate the extent to which departments themselves are Europeanized by the EU, or the impact that such a change could have on national policy. Taking as an example the Europeanization of one policy sector (the environment) in the United Kingdom, this article reveals that the national department has undergone a much deeper and more profound cultural change than one would expect if national structures were essentially unchanged by the Europeanization of policy. This single, but critical, case implies that the next phase of Europeanization research should open up the 'black box' of the state and examine the co-evolution of administrative structures, policies and politics.