Through the transformation of undergraduate STEM courses, the Colorado Learning Assistant Program recruits and prepares talented STEM majors for careers in teaching by providing them with early, sustained teaching experiences. The research reported here compares teaching practices of K-12 teachers who served as learning assistants (LAs) as undergraduates to colleagues that were certified through the same teacher certification program but did not serve as LAs. Observations of teacher practices revealed that former LAs used significantly more reformed teaching practices than their colleagues, especially in their first year of teaching. These results suggest the LA Program serves as a valuable supplement to traditional teacher certification programs.