We tested the effectiveness of a course-long intervention in an undergraduate Research Methods course aimed toward reducing students' endorsement of hostile sexism (HS) and benevolent sexism (BS). Reading assignments illustrating diverse research methodologies, lecture examples, and a hands-on research project designed by student teams focused on ambivalent sexism--a topic that has personal relevance for students as well as research findings documenting its harmful individual, interpersonal, and sociopolitical effects. Of the 101 students across three Methods courses taught at a U.S. Midwestern comprehensive university by the same instructor, 31 responded to a postintervention survey, showing significant declines in HS and BS that were not duplicated in a comparison group of 29 twice-tested psychology majors. Supplemental analyses ruled out selection biases between responding and nonresponding intervention students and between all intervention students and 55 comparison psychology majors tested at the start of their courses. Our findings from a quasi-experiment in a naturalistic setting do suggest that repeated exposure to theory and research about ambivalent sexism can favorably influence students' attitudes--thus adding a feminist social justice agenda targeting psychology majors as well as a model for targeting students' other prejudices.