This paper researches the effects that students' metacognitive knowledge has on their participation in online forum discussions, which form part of a web-based asynchronous course based on a constructivist instructional approach. Metacognitive knowledge increases learners' ability to be independent learners, which is an indispensable characteristic of the distant learner. The study was carried out with 32 third-grade pre-service teachers. Each message in the forum discussions was analyzed in terms of interaction types identified by McKinnon (2000) and also scored using a grading rubric developed by the researchers. The metacognitive knowledge of the pre-service teachers was measured by the component of the General Metacognition Questionnaire. Sixty-seven percent of the pre-service teachers were at the high or medium-to-high metacognitive knowledge level and mostly sent messages having "example to idea," "clarification and elaboration," or "idea to example" type interactions. Pre-service teachers who exhibited low metacognitive knowledge, however, mostly forwarded messages having "acknowledgments," "unsubstantiated judgment," or "thoughtful query" type interactions. Moreover, metacognitive knowledge of the pre-service teachers uniquely explained 21.4% of the variance in the online participation score. We concluded by outlining some implications metacognitive knowledge has on forum discussions in relation to the constructivist approach. (Contains 1 figure and 6 tables.)