Attrition from the sciences remains a national problem. The authors present results from a survey of over 2,100 undergraduates that, contrary to previous research, suggests that teaching assistants (TAs) influence student retention in the sciences in multiple ways. Multiple linear regression (MLR) and student comments suggest that TAs influence lab climate, course grades, and students' knowledge of science careers, all of which have an effect on students' decisions to stay in or leave the sciences. The authors close with recommendations for TA training, mentoring, and management to positively impact student retention. (Contains 2 tables.)