Social identity threat has negative consequences for women in science, technology, engineering, and mathematics (STEM) fields. The present study examined whether legitimacy beliefs--beliefs that status differences between men and women in STEM fields are fair--put women at risk for experiencing social identity threat and poorer performance on a difficult logic test. Legitimacy beliefs served as a risk factor for both women from ethnic groups that are overrepresented in STEM (e.g., Asian Americans and European Americans) and from ethnic groups that are underrepresented in STEM (African Americans and Latina Americans), albeit under different conditions. Among women from overrepresented ethnic groups, legitimacy beliefs were negatively related to test performance when explicit cues to sexism were present. However, among women from underrepresented ethnic groups, legitimacy beliefs were negatively related to test performance when explicit cues to sexism were absent. The present research points to the need for inclusion of ethnic diversity in studies of women in STEM.