The high attrition rate among science, technology, engineering, and mathematics (STEM) majors has long been an area of concern for institutions and educational researchers. The transition from introductory to advanced courses has been identified as a particularly "leaky" point along the STEM pipeline, and students who struggle early in an introductory STEM course are predominantly at risk. Peer-tutoring programs offered to all students in a course have been widely found to help STEM students during this critical transition, but hiring a sufficient number of tutors may not be an option for some institutions. As an alternative, this study examines the viability of an optional peer-tutoring program offered to students who are struggling in a large-enrollment, introductory biology course. Struggling students who regularly attended peer tutoring increased exam performance, expert-like perceptions of biology, and course persistence relative to their struggling peers who were not attending the peer-tutoring sessions. The results of this study provide information to instructors who want to design targeted academic assistance for students who are struggling in introductory courses.