One way to frame science classroom discussion is to engage students in scientific argumentation, an important discourse format within science aimed at coordinating empirical evidence and scientific theory. Framing discussion as scientific argumentation gives clear priority to contributions that are sustained by evidence. We question whether this priority is conducive to conceptually rich student talk (talk in which students elaborate key concepts and causal mechanisms). Coding transcripts of six middle school classrooms engaged in whole-class scientific argumentation, we identified whether student conversations about a physical science concept incorporated arguments that were supported by evidence and whether the same conversations amounted to conceptually rich talk. Rich talk and evidence-supported arguments rarely occurred together in the same conversation. In a detailed analysis of selected conversations, we argue that the priority given to evidence within scientific argumentation incurs constraints on discussion goals and reasoning that tend to inhibit conceptually rich talk. (Contains 13 tables and 1 figure.)