National calls for improving science education (e.g., "Vision and Change") emphasize the need to learn disciplinary core ideas through scientific practices. To address this need, we engaged small groups of students in developing diagrammatic models within two (one large-enrollment and one medium-enrollment) undergraduate introductory biology courses. During these activities, students developed scientific models of biological phenomena such as enhanced growth in genetically modified fish. To investigate whether undergraduate students productively engaged in scientific practices during these modeling activities, we recorded groups of students as they developed models and examined three characteristics: how students (1) interacted with one another, (2) made sense of phenomena, and (3) justified their ideas. Our analysis indicates that students spent most of the time on task, developing and evaluating their models. Moreover, they worked cooperatively to make sense of core ideas and justified their ideas to one another throughout the activities. These results demonstrate that, when provided with the opportunity to develop models during class, students in large-enrollment lecture courses can productively engage in scientific practices. We discuss potential reasons for these outcomes and suggest areas of future research to continue advancing knowledge regarding engaging students in scientific practices in large-enrollment lecture courses.