Many undergraduates who wish to pursue degrees in science, particularly students from underrepresented groups, drop out of science majors before realizing their goal. This study examines the effectiveness of a mentoring program — called Biomentors — aimed at promoting success in biology courses for undergraduates beginning their coursework toward a bachelor's degree in the biological sciences. Students enrolled in the Biomentors program met twice a week in a small group with an advanced biology major under the supervision of a faculty member to explore effective learning strategies for success in an introductory-level biology course they were taking. Students who participated in the Biomentors program scored significantly higher (based on total points earned) than other students enrolled in the course across two cohorts (d = 0.36 in the fall quarter of 2014; d = 0.34 in the winter quarter of 2015). The biomentors group significantly outscored the control group even when the effects of gender, parent income level, parent education level, total SAT score, and cumulative GPA were statistically controlled using a stepwise regression. Overall, the results encourage further investigation of the effectiveness of peer-mentoring programs that emphasize domain-specific learning strategies for college students beginning as science majors.