Seligman's preparedness theory of phobia is tested in human classical conditioning of skin conductance and heart rate responses. Conditioned stimuli (CSs) were photographs of plants, human artifacts, and phobia-relevant animals. Both aversive tactile and auditory unconditioned stimuli (UCSs) were assessed. Consistent with Öhman's results, electrodermal extinction was slower to phobia-relevant stimuli than to other stimuli (and here, was shown to depend on a tactile UCS); however, unlike Öhman's findings, when subjects were told "no more shocks," phobic CSs extinguished as readily as unprepared CSs. New evidence was obtained for a preparedness effect during acquisition trials: Only subjects receiving phobia-relevant stimuli developed an acceleratory cardiac conditioned response. This acquisition effect was more reliable across experiments than the electrodermal extinction findings and showed less influence of CS-UCS "belongingness." These experiments suggest that the preparedness effect is complexly determined; they also provide clear evidence that phobic stimuli occasion a unique pattern of conditioned visceral response. © 1986 American Psychological Association.