This case study investigated patterns of interaction and participation in a large online course. 88 Korean undergraduates participated in online fora during 2 weeks. It was found that there was a comparatively high portion of metacognitive interaction and higher phase of knowledge construction. However, it was found that students posted required number of messages at once around due date and selectively responded to peers' initiations without turn-taking. There were few responses despite many readings of peers' postings to meet the required number and type of posting. The results showed that the discussion question and evaluation criteria influenced patterns of interaction and participation, and the phase of knowledge construction. Online instructional strategies were suggested for successful online discussion, especially to achieve sustainable discussion. (Contains 8 tables and 3 figures.)