Peer-tutoring programs have been found to produce positive effects on student achievement. There has been relatively little research on the effects of tutor characteristics on the achievement of college students who receive tutoring. The purpose of this study was to investigate the relationship between the sex of tutors and students' sex on achievement in introductory-level science and math courses for students who received tutoring. The grades of 333 students were evaluated using analysis of variance. The results of this study indicate that students did better when they worked with a tutor of the same sex. The availability of same-sex tutors may provide a strategy for improving female students' achievement in college math and science courses. (Author)