Graduate teaching assistants (GTAs) are responsible for teaching the majority of biology undergraduate laboratory sections, although many feel underprepared to do so. This study explored the impact of biology GTA participation in a professional development model known as lesson study. Using a case study methodology with multiple qualitative data sources, this study found that lesson study was beneficial for this group of GTAs in that it modified critical aspects of their beliefs about biology instruction. Each participant felt that lesson study helped revise their teaching and changes were seen in some aspects of the participants' Pedagogical Content Knowledge (PCK). Despite this, there was an observed disconnect between participants' vocalized intent and classroom practice. This disconnect could be attributed to the difficulty of implementing new strategies, the short duration of the lesson study, and the instructional inexperience of the participants in the study.