This article analyses the national transposition process of EC directives in the member states of the European Union. We ask why national governments almost exclusively choose using executive instruments for the transposition of EC directives and test a prominent view for the in- respectively exclusion of their domestic parliaments - do national governments prefer gathering bureaucratic information about the implementation of EC law in order to reduce their uncertainty about policy outcome, or, do they use parliamentary inclusion to avoid possible bureaucratic drifts? The tested approach promotes an ex post-perspective on implementation problems which builds on insights of the delegation literature on transaction costs predicting parliamentary exclusion when the governmental majority’s preference is cohesive and uncertainty about the directive’s implementation effects is high. Using a comprehensive dataset on all notified transposition measures in 15 member states for EC directives initiated by the Commission between 1986 and 2002, our multi-level analysis shows that national governments are more likely to use parliamentary transposition measures for particular important directives, decided under the unanimity voting rule in the Council. On closer inspection, our analysis reveals that the principal agent problem between a coalition government and the responsible minister also impacts the transposition process because governments prefer parliamentary transposition measures when this problem is severe. Tertiary legislation, which is solely decided by the Commission, is significantly more likely to be transposed by executive measures than any other type of legislation.