Spider phobics were tested before and after one-session treatment for spider phobia, or a comparable waiting period, using a spider-word Stroop test and questionnaires in which they rated spider-relevant threat beliefs. Compared with untreated spider phobic controls, the treated phobics changed significantly in their negative beliefs about spiders after treatment. Controls and treated phobics showed the same change in their reaction time latencies to spider stimuli in the Stroop test. These data are consistent with the hypothesis that the modification of threat beliefs is crucial in changing the response to phobic stimuli. It is concluded that the threat-specific Stroop test is an ambiguous measure of fear-related cognitive processes.