This study explored defensive response mobilization as well as fMRI responses during sustained exposure to phobia-relevant stimuli. To test the specificity of affective physiology and brain activation, neutral and other affective stimuli were included. Phobia-specific startle potentiation was maintained and autonomic responses even increased during sustained phobic stimulation. Viewing of spider pictures also resulted in increased activation of the amygdala in spider-phobic participants. This effect, however, was not fear specific because other affective materials evoked comparable signal strength in the amygdala. In contrast, insula activation was specifically increased during sustained phobic exposure in phobic volunteers. These data suggest that the activation of the amygdala in fMRI studies primarily indexes the detection of motivationally relevant stimuli whereas the insula might be more specifically linked to defensive response mobilization.