Abstract. This study analyzes party group cohesion and patterns of defections of national party delegations from party group lines in the European Parliament (EP), using a total of 2,582 roll call votes. The study confirms previous findings according to which party groups in the EP show (surprisingly) high levels of cohesion. Nevertheless, it reveals the circumstances under which Members of the EP (MEPs) and their national delegations are more likely to defect, using the candidate selection process, the electoral system and relationships between MEPs and their home parties as explanatory variables. Assuming that MEPs have three different goals (re-election, office and policy), and want above all to secure re-election, one can expect that those MEPs whose chances of re-election are more dependent on national parties than others are more willing to vote against the party group line if a conflict between party group and national party emerges. Empirically, this is confirmed.