Many researchers have noted the similarities between causal judgment in humans and Pavlovian conditioning in animals. One recently noted discrepancy between these two forms of learning is the absence of backward blocking in animals, in contrast with its occurrence in human causality Judgment. Here we report two experiments that investigated the role of biological significance in backward Mocking as a potential explanation of this discrepancy. With rats as subjects, we used sensory preconditioning and second-order conditioning procedures, which allowed the to-be-blocked cue to retain low biological significance during training for some animals, but not for others. Backward blocking was observed only when the target cue was of low biological significance during training. These results suggest that the apparent discrepancy between human causal judgment and animal Pavlovian conditioning arises not because of a species difference, but because human causality studies ordinarily use stimuli of low biological significance, whereas animal Pavlovian studies ordinarily use stimuli of high biological significance, which are apparently protected against cue competition.