Students often hold strong attitudes regarding topics they encounter during their studies, and many instructors feel that these attitudes can have strong effects on students' performance. We characterized students' attitudes toward evolution and investigated the influence of students' attitudes (pre-course and post-course) regarding evolution on their performance in an evolution course, measured as their final grade. We found our students to hold positive attitudes toward evolution; these attitudes became more positive following the course. The most significant change in attitude occurred in the group of students initially undecided toward evolution. We also found that attitudes prior to the course had little influence on later achievement; however, at the end of the course, students' attitudes were positively related to final grades, although the effect was small. We argue that pedagogical techniques directly addressing students' attitudes help reduce the influence of attitudes (especially prior attitudes) on achievement. © 2005 Wiley Periodicals, Inc. J Res Sci Teach 43: 7–24, 2006