The stereotype threat theory of C. Steele (1992, 1997) attempts to explain the underperformance of minority students in academic domains and of women in mathematics. Steele asserts that situational self-relevance of negative group stereotypes in testing situations increases the anxiety these students experience, and that these differential anxiety levels explain performance differences. Research shows that manipulation of stereotype threat can affect academic performance. However, there has been little research testing whether anxiety does at least partially explain the relationship between race and achievement. The goal of this study was to examine whether anxiety will explain racial differences in academic performance and gender differences in mathematics performance in the context of a nationally representative sample of high school seniors. Data were drawn from the senior cohort data file of the High School and Beyond study, a cohort that initially consisted of 28,240 seniors from 1,015 schools. Partial mediation was observed, with anxiety explaining significant portions of the racial differences in academic performance. Anxiety also partially explained sex differences in mathematics achievement, although the effect sizes were very small. These results provide general support for Steele's stereotype threat hypothesis. (Contains 3 tables and 54 references.) (Author/SLD)