Normal subjects (n = 64) were exposed either to pictures of snakes and spiders or to pictures of flowers and mushrooms in a differential conditioning paradigm in which one of the pictures signaled an electric shock. In a subsequent extinction series, these stimuli were presented backwardly masked by another stimulus for half of the subjects, whereas the other half received non-masked extinction. In support of a hypothesis that suggests that nonconscious information-processing mechanisms are sufficient to activate responses to fear-relevant stimuli, differential skin conductance response to masked conditioning and control stimuli was obvious only for subjects conditioned to fear-relevant stimuli. These results were replicated in a second experiment (n = 32), which also demonstrated that the effect was unaffected by which visual half-field was used for stimulus presentation.