To promote rich discourse around scientific and engineering practices, teachers may turn to engineering design-based science curricula; however, this has discursive demands which have yet to be examined in a unit focused on integration of engineering and science. To investigate these discursive demands, we expand on the definition of discourse to include the ways of knowing, doing, talking, reading, writing, and context within science and engineering--and refer to these as disciplinary Discourses. The following major research question guided this study: how are multiple Discourses enacted by both the teacher during whole class discussions and students in small groups in an engineering design-based curricular unit? In this descriptive case study, we investigated these disciplinary Discourses in a design-based unit that focused on integration of engineering into a sixth-grade genetics unit through whole-class videos and audio recordings of a target student group. Our findings suggest that while the Discourses present in whole class discussions often reflected the focus of the daily lesson, this unit creates a space for multidisciplinary Discourse to emerge between science and engineering. Also, this teacher merged everyday and more technical disciplinary Discourses to scaffold the students' understanding. Finally, we observed multidisciplinary Discourses between science and engineering emerge in our student group, suggesting that students can successfully integrate the two disciplines. This study therefore begins to investigate the disciplinary Discourses present in a design-based curricular unit and specifically multidisciplinary Discourses that may enhance students' understanding of both science and engineering.