Creating environments in which students are engaged in disciplinary work has long been a goal for many teachers and education reformers but has proven difficult to create and sustain. In such environments students are envisioned to be actively and publicly engaged in their learning while utilizing disciplinary language, ideas, and methods. This case explores a specific series of discussions in an elementary school that utilized disciplinary thinking (Engle & Conant 2002; Engle 2006; Engle, Conant & Greeno, 2007). While participating in a curriculum aimed at fostering science reasoning, content literacy, and creating student learning communities, two groups of fifth-graders took part in an ongoing informal debate over the taxonomical classification of orcas, or killer whales. The debate centered on whether orcas were properly classified as whales or dolphins. According to researchers who documented this activity, instances in which the students took part in the debate were marked by heightened levels of attention, engagement, and emotionality. Eight such instances of debate over the classification of orcas were observed by the researchers. Summaries of all eight are presented in this case, but the primary focus is on one specific instance where four students described the controversy to a new student teacher. More specifically, this case will describe the curriculum and structure of tasks utilized by the teachers, aspects of the social dynamics between students, and empirical evidence for how the instances of argumentation influenced the students’ level of engagement.