Assigning roles to individual students can influence the group's knowledge construction (KC) process during online discussions. Twenty-one students were divided into two groups and assigned rotating roles for eight one-week asynchronous online discussions. The KC contributions of all 252 posts in the discussion were coded using a five phase scheme and statistical discourse analysis was applied to identify segments of discussion characterized by particular aspects of KC plus "pivotal posts"-those posts which initiated new segments of discussion. Finally, the influences of assigned student roles on pivotal posts and KC were modeled. The results indicate that most online discussions had a single pivotal post separating the discussion into two distinct segments: the first dominated by a lower KC phase; the second dominated by a higher KC phase. The pivotal posts that initiated later segments were often contributed mid-discussion by students playing one of two summarizing roles (Synthesizer and Wrapper).