This study examined the effects of pre-structuring discussion threads on group performance in computer-supported collaborative argumentation where students labeled their messages as arguments, challenges, supporting evidence, and explanations on a threaded discussion board. In the pre-structured group students were required to post supporting and opposing arguments to designated and separate discussion threads. In the control group no discussion threads were designated for posting supporting and opposing arguments. The mean number of challenges elicited per argument was 64% greater with pre-structured threads (ES = +0.47). No significant differences were found in the mean number of counter-challenges, supporting evidence, and explanations posted in reply to challenges. The findings suggest that prestructured discussion threads can increase the frequency of argument[right arrow]challenge exchanges needed to initiate critical discourse. (Contains 1 table and 6 figures.)