Members of the European Parliament (MEPs) have a multitude of parliamentary duties and, accordingly, have to prioritize some parliamentary activities over others. So far, we know comparably little about this prioritization process. Based on principal–agent theory, we argue first, that MEPs’ parliamentary activities are systematically determined by the “visibility” and usefulness of parliamentary instruments for their key principal; second, we expect the exclusiveness of candidate selection procedures of an MEP’s national party—the nomination and the final list placement—to determine her/his key principal (i.e., elites or members of national parties). Combining multi-level mixed effects linear regression models and expert interviews, we show that MEPs who are nominated and whose final list placement is decided by an exclusive circle of national party elites prioritize speeches, whereas MEPs who are nominated or whose final list placement is decided by more inclusive procedures prioritize written questions and opinions or reports. In other words, speeches seem particularly useful to communicate with national party elites, while other activities are used to serve larger groups of party members. These findings open up the black-box of the “national party principal” and illustrate how a complex principal–agent relationship stimulates very specific parliamentary activity patterns in the EU’s multi-level system.