This study investigated the role of negative thinking as a potential mediator of performance deficits under stereotype threat. After being assigned to a stereotype-threat or a no-threat condition, 60 female participants were asked to complete a difficult math task. Using the thought-listing technique, women under stereotype threat reported a higher number of negative thoughts specifically related to the test and to mathematics compared with women in the no-threat condition. Moreover, women under stereotype threat also showed a sharp decrease in performance that (a) was most pronounced in the second half of the test and (b) was mediated by the increase in negative thinking.