Argumentation, and the production of scientific arguments are critical elements of inquiry that are necessary for helping students become scientifically literate through engaging them in constructing and critiquing ideas. This case study employed a mixed methods research design to examine the development in 5th grade students' practices of oral and written argumentation from one unit to another over 16 weeks utilizing the science writing heuristic approach. Data sources included five rounds of whole-class discussion focused on group presentations of arguments that occurred over eleven class periods; students' group writings; interviews with six target students and the teacher; and the researcher's field notes. The results revealed five salient trends in students' development of oral and written argumentative practices over time: (1) Students came to use more critique components as they participated in more rounds of whole-class discussion focused on group presentations of arguments; (2) by challenging each other's arguments, students came to focus on the coherence of the argument and the quality of evidence; (3) students came to use evidence to defend, support, and reject arguments; (4) the quality of students' writing continuously improved over time; and (5) students connected oral argument skills to written argument skills as they had opportunities to revise their writing after debating and developed awareness of the usefulness of critique from peers. Given the development in oral argumentative practices and the quality of written arguments over time, this study indicates that students' development of oral and written argumentative practices is positively related to each other. This study suggests that argumentative practices should be framed through both a social and epistemic understanding of argument-utilizing talk and writing as vehicles to create norms of these complex practices.