Current science education research and policy highlight the need to conceptualize scientific disciplines not only based on a view of “science-as-knowledge” but also on a perspective of “science-as-practice,” placing an emphasis on practices such as explanation, argumentation, modeling, and communication. However, classroom discourse is not structured in a way that would normally provide to students the opportunity to engage in such “dialogic knowledge-building processes” (Duschl, 2008a) or epistemic discourse. This study argues that such a change in classroom discourse can be achieved through a focus on argumentation as an instructional approach, which aims to engage students in the epistemic practices of science. This study focuses on a qualitative case study of an experienced teacher's attempts to use argumentation over a school year as a way to identify elements of epistemic discourse that science teachers could be making part of their everyday science teaching. The analysis of classroom talk focused on (a) the teacher's discursive actions or epistemic operations, and (b) the ways in which these discursive actions presented or engaged students in the construction, justification, and evaluation of knowledge claims. The analysis revealed that the use of justificatory talk was consistent across the six lessons observed but the same consistency was not identified in attempts to engage students in evaluative practices. This discrepancy would suggest that evaluative practices were not as embedded in the teacher's classroom talk as the justification or construction of knowledge claims. Implications discussed include the need to reconsider pre-service and in-service teacher training and professional development so that science teachers do not only develop their skills of teaching science based on argument, but also of talking science based on argument. © 2014 Wiley Periodicals, Inc. J Res Sci Teach 51: 1275–1300, 2014.