Robert Carson's principle of complementarity asserts that the behavioral styles of interaction partners tend to complement each other by encouraging individuals to act opposite in terms of dominance and similar in terms of warmth. The principles of complementarity further hypothesize that as relationships progress through multiple interactions, the behavioral styles of its members will be altered to increase complementarity. To examine this acquaintanceship hypothesis, the behavioral styles and personalities of 102 college roommate dyads were assessed after living together for 2 weeks and again after living together for 15 weeks. Consistent with the acquaintanceship hypothesis, after 2 weeks the behavioral styles of roommates did not complement each other; however, after 15 weeks, the behavioral styles of roommates strongly complemented each other. In contrast to the change in complementarity observed in roommates' behavioral styles, participants' perceptions of their own personalities were relatively unaffected by the personalities of their roommates.