This study examines how grade 5 students engage with the aspects (construct and critique) of argument in an online asynchronous discussion combined with in-class wrap-up discussion. Grade 5 students in a rural public school engaged in a "human health investigation" unit using an argument-based inquiry approach followed by online asynchronous discussions using a Moodle forum. The online discussion was wrapped up by a 1-h in-class discussion and student writing of claims, evidence, and reflection. Data sources included online notes posted by 111 students in the Moodle forum, a video record of in-class discussion, and writing samples of claims, evidence, and reflections produced by 54 students after online and in-class discussions. Results of this study indicate that students engaged with the construct and critique components of argument in the online asynchronous discussion and in the in-class wrap-up discussion. The results show that in terms of the argument components they (a) used evidence resources to challenge and supplement arguments, (b) critiqued and reinforced evidence, and (c) strongly negotiated and confirmed/revised claims.