Laboratory graduate teaching assistants (GTAs) play a prominent role in undergraduate chemistry education. Although the success of a laboratory program relies significantly on the performance of GTAs, only rarely have they been considered actual partners in instruction or have their experiences in the academic lab been investigated. This paper reports on an embedded multiple case study designed to investigate how chemistry graduate students in two independent and very different learning environments constructed their GTA self-image and what aspects contributed to this process. Participants in this study included 13 GTAs from an expository-based program and 11 GTAs from an inquiry-based program. Interviews, memos, training artifacts, laboratory manuals, and course syllabi were collected to generate case reports for each program. Findings suggest that GTAs' construction of their self-image is shaped through the interaction of several factors: prior experiences, training, beliefs about the nature of knowledge and about the nature of academic laboratory work, and involvement in the laboratory setting. Findings from this study encourage laboratory coordinators to reconsider GTA participation in instruction in a new and different light and, when applicable, to look at their training and support from a new angle. Instead of focusing exclusively on what and how to teach, lab coordinators may design training and support programs that also target the factors that influence construction of GTA self-image. A more fruitful experience is attainable for students and GTAs insofar as the graduate students develop a teaching assistant self-image consistent with the specific instructional objectives.