This study examined the extent to which fifth-grade students participate in online argumentation and the argument patterns they produced about the inquiry-based investigations completed using the Science Writing Heuristic approach in their science classes. One hundred twenty-nine students from five classes of two teachers in a Midwestern public school completed two inquiry-based investigation units, one per semester, followed by asynchronous online discussions using the Moodle forum. Among the 129 students, 107 students produced 739 notes in the plant investigation online discussion and 111 students produced 686 notes in the human health investigation online discussion. Results indicate that students were actively engaged in the online discussions about inquiry investigations with comments being focused on providing more evidence and backing for claims and negotiating evidence in both investigations. The students also engaged in challenging and querying the test procedures and reference sources as the basis for evidence. Implications are discussed for science teaching and learning and further study on argument-based inquiry in online environments.