Because source use is a key academic literacy skill tied to students’ socialization into the university, scholars have called for more research on how novice second language (L2) writers’ use of sources changes over time as they engage with disciplinary discourse. The present study, therefore, tracked the semester-long development of thirty undergraduate L2 students’ source use in a research writing seminar course. Each student wrote two research papers for the course, providing sixty papers for both quantitative and qualitative text analysis. The researcher conducted data analysis in terms of citation density, source type, citation type, and source use purpose. Findings showed that students’ engagement with scholarly articles led to formulation of new citation patterns: incorporation of research summaries and frequent use of nonintegral citations. In addition, citation density increased overall, with scholarly sources newly used in theoretical orientations to John M. Swales’s CARS model. Nonetheless, students’ papers demonstrated a lack of proficiency in the sophisticated aspects of source use. The discussion concludes with suggestions for source use instruction in line with students’ understanding of disciplinary discourse.