This study investigated how congruence between dyadic partners' leader and follower prototypes affects leader-member exchange (LMX) quality. Specifically, we examined cooperation as a process variable in the dyadic relationship. Participants in a laboratory setting completed a group task followed by dyadic task in the context of a leader-follower relationship. Observed cooperation mediated the relationship between congruence on leader prototypes and leader assessed LMX quality, and the relationship between congruence on leader prototypes and LMX agreement. As congruence on leader prototypes decreased, leaders were less likely to be cooperative in an exchange relationship. As congruence on follower prototypes decreased, there was a greater chance leaders would cooperate but followers would defect.