Thirty-seven graduate students in two course sections were required to contribute to the online discussion of an instructional design case study. The discussion forums required different participation protocols; students in section one implemented a constructive argumentation approach while students in section two had less structure for their postings. The results revealed that both sections exhibited co-construction of knowledge as measured using the content analysis protocol the Interaction Analysis Model (IAM), but the less structured section reached the highest phase of knowledge building. In addition we found that the constructive argumentation approach and the task-oriented nature of the discussion influenced the interactions and co-construction of knowledge. The findings reveal practical implications for effective design of online discussion that may improve the quality of learning.