Drawing on sociological and philosophical studies of science, science educators have begun to view argumentation as a central scientific practice that students should learn. In this article, we extend recent work to understand the structure of students' arguments to include judgments about their quality through content analyses of high school students' written explanations for 2 problems of natural selection. In these analyses, we aim to explicate the relations between students' conceptual understanding of specific domains and their epistemic understanding of scientific practices of argumentation as they try to learn science through inquiry. We present a method that assesses the warrant of explanatory claims, the sufficiency of the evidence explicitly cited for claims, and students' rhetorical use of specific inscriptions in their arguments. Students were attentive to the need to cite data, yet they often failed to cite sufficient evidence for claims. Students' references to specific inscriptions in their arguments often failed to articulate how specific data related to particular claims. We discuss these patterns of data citation in terms of what they suggest about students' epistemological ideas about explanation and consequent implications for inquiry-oriented, science education reforms.