Human conditioning research has revealed an apparent resistance to extinction of aversive conditioning to pictures of fear-relevant stimuli such as snakes and spiders, supporting M. E. P. Seligman's (1971) preparedness theory of fears and phobias. This article examines an alternative account based on activation of preexisting response tendencies under threat (selective sensitization). Two experiments demonstrate that selective sensitization of electrodermal responses is attenuated when a fear-relevant stimulus serves as a negative conditioned stimulus (CS−), but is maintained when it serves as a positive conditioned stimulus (CS+). Previous extinction results may therefore be due to preservation of initial responding to CS+ but not CS−. Selective sensitization offers a model for the nonassociative activation of fears and phobias to prepotent stimuli under conditions of stress or threat. Possible genetic and cognitive mechanisms are discussed.