This paper studies party discipline in a congress within a political agency framework with retrospective voting. Party discipline serves as an incentive device to induce office- motivated congress members to perform in line with the party leadership's objective of controlling both the executive and the legislative branches of government. I show fi rst that the same party is more likely to control both branches of government (i.e., uni ed government) the stronger the party discipline in the congress is. Second, the leader of the governing party imposes more party discipline under uni ed government than does the opposition leader under divided government. Moreover, the incumbents' aggregate performance increases with party discipline, so a representative voter becomes better off. JEL classi cation: D72. Keywords: Party discipline; Political agency; Retrospective voting; Office-motivated politicians.