Research shows that scientific knowledge develops through a process of decision-making as well as discovery, and that argumentation is a genre of discourse crucial to the practice of science. Students should therefore be supported in understanding the scientific practices of dialectical and rhetorical argumentation as part of learning about scientific inquiry. This study focuses on supporting scientific argumentation in the classroom through a customized online discourse system. "Personally-seeded discussions" support learning and collaboration through an activity structure that elicits, shares, and contrasts students' own ideas to engage them in the discourse of science argumentation and inquiry. Students use an online interface to build principles to describe data they have collected. These principles become the seed comments for the online discussions. The software sorts students into discussion groups with students who have built different principles so that each discussion group can consider and critique multiple perspectives. This study explores the efficacy of this personally-seeded approach based on a coding scheme developed by Erduran, Osborne, and Simon that analyzes argument structure from a Toulmin perspective. As part of this exploration, the study outlines a method for parsing personally-seeded discussions into oppositional episodes for analysis, and discusses future directions for supporting argumentation in asynchronous online discussions. (Contains 4 figures and 2 tables.)