Accession of the Central and Eastern European countries to the European Union involved considerable institutional changes in the field of environmental governance that undermined existing policy practices. Such changes in biodiversity governance are examined through two case studies from Poland concerning road development schemes in ecologically sensitive areas. Documentary material and semi-structured interviews are analysed and interpreted in light of the policy arrangement approach and historical institutionalism. After the EU accession there was a rapid shift from a government-monopolized arrangement towards a multi-level governance arrangement with regard to conflicts between infrastructure and natural areas. Previously, the government controlled practically all aspects of environmental policymaking, despite potential routes for greater involvement of non-state actors. Only the formal rules of Natura 2000 and environmental impact assessment, guarded by a coalition of powerful supranational bodies, non-governmental organizations and scientists, changed the pattern of governmental policy-making. New policy arrangements have increased capacity and legitimacy in the implementation of the European Union rules.