The role of d subunit-containing GABA[subscript A] receptor (GABA[subscript A](d)R) in fear generalization is uncertain. Here, by using mice with or without genetic deletion of GABA[subscript A](d)R and using protocols in which the conditioned tone stimuli were cross presented with different nonconditioned stimuli, we observed that when the two tone stimuli were largely similar, both genotypes froze similarly to either of them. However, when they differed markedly, the knockout mice froze much more than their wild-type littermates to the nonconditioned but not conditioned stimuli. Thus, GABA[subscript A](d)R may prevent inappropriate fear generalization when the incoming stimuli differ clearly from the learned threat.