Subjects in six experimental groups (n = 16 each) received one-trial passive avoidance (PA) training in which shock was delivered upon movement from a white wooden floor compartment to a black grid compartment. Then fear was extinguished (30 min) in the black compartment. After either 24 or 168 h, all the groups were treated in a room distinctively different from the training room. At each interval, one group received a shock in an apparatus similar to the conditioning box, another received a shock in a dissimilar apparatus, and another was placed in a neutral box. A PA test trial in the training apparatus indicated reinstatement of extinguished fear in all the groups given a postextinction shock except the 24-h dissimilar group. Control groups revealed that the extinction treatment was effective and that spontaneous recovery was not evident. The results were explained in terms of classical conditioning, stimulus generalization, and the broadening (flattening) of stimulus generalization gradients with time.