This study investigated friend influence over mathematics achievement in 202 same-sex friendship dyads (106 girl dyads). Participants were in the third grade (around age 9) at the outset. Each friend completed a questionnaire describing interest in mathematics and a standardized mathematical reasoning assessment. Peer nominations provided a measure of peer acceptance. The results revealed evidence that interest in mathematics moderates both the degree to which the higher-accepted friend was influential and the degree to which the lower-accepted friend was susceptible to influence. Specifically, the third-grade mathematical reasoning of the higher-accepted friend predicted an increase in the mathematical reasoning of the lower-accepted friend from third grade to fourth grade only when one friend was above average on interest in mathematics. These effects held after controlling for maternal support, parental education, friendship duration, friendship group norms, friend similarity on peer acceptance, friend similarity on interest in mathematics, and general academic functioning.