Graduate teaching assistants (GTAs) are gaining increasing responsibility for the instruction of undergraduate science students, yet little is known about their beliefs about science pedagogy or subsequent classroom practices. This study looked at six GTAs who were primary instructors in an introductory biology laboratory course. Teaching assistants taught a lesson about the potential social, health, and environmental impacts of genetically modified crops. Through classroom observations and in-depth interviews, the researchers examined how instructors chose to frame their lessons and what GTAs perceived as important for students to know about this particular socioscientific issue (SSI). Results showed a disconnect between the relatively mature conceptualizations of effective SSI instruction that emerged during interviews and classroom practice. (Contains 3 tables.)