Although research has shown that priming negative stereotypes leads to lower performance among stigmatized individuals, little is understood about the cognitive mechanism that accounts for these effects. Three experiments tested the hypothesis that stereotype threat interferes with test performance because it reduces individuals' working memory capacity. Results show that priming self-relevant negative stereotypes reduces women's (Experiment 1) and Latinos' (Experiment 2) working memory capacity. The final study revealed that a reduction in working memory capacity mediates the effect of stereotype threat on women's math performance (Experiment 3). Implications for future research on stereotype threat and working memory are discussed.