Graduate teaching assistants (GTAs) teach many undergraduates in introductory science courses in the USA. Previous literature suggests that GTAs would prefer more context-dependent opportunities to learn to teach more effectively. This study explores the impact of participation in a lesson study cycle with GTAs responsible for an undergraduate introductory course in Earth Science. Data were analyzed via a theoretical perspective of situated cognition and lesson study. Findings indicate GTAs felt limited by current contexts in the degree to which they could change the laboratory, their talk about teaching largely focused on logistical issues, yet via the lesson study process they began to discuss the learner in their seminars. Implications are presented that explore the applicability of lesson study in this context, the role of a science educator as a facilitator for reform in college science teaching, and the potential impact of this process on GTAs' relationships with other faculty. (Contains 1 table.)