Research on multi-level lobbying suggests that interest group mobilization differs between levels of government. Assuming differential abilities of interest groups to overcome collective action problems, we argue that business interests are over-represented in particular at higher levels of the political system. In order to assess the size and direction of business bias in interest representation, the distributions of actor types at different levels have to be mapped and compared. In this paper, we will map interest group populations on the subnational, national, and European level and their mobilization across different policy areas. Our analysis starts with the subnational level by mapping all groups based in the German state of Baden-Württemberg. To determine the set of active organizations on the three levels, we then trace which of these interest groups are present in the respective lobby and transparency registers in order to capture groups’ political mobilization. Controlling for delegation to umbrella groups and direct corporate lobbying, our analysis will address a lack of attention to multilevel dynamics in interest group populations and yield important insight in the equality of political representation.