We studied eight populations of the red-legged frog, Rana aurora, to examine responses of allotopic and syntopic tadpoles to the bullfrog, Rana catesbeiana, an introduced predator of R. aurora. We also assessed predation rates by R. catesbeiana on syntopic and allotopic populations of R. aurora. Syntopic R. aurora tadpoles significantly reduced their activity and increased their refuge use when presented with the chemical cues of both tadpoles and adult R. catesbeiana. In contrast, allotopic tadpoles did not significantly alter their behavior in the presence of either R. catesbeiana adults or larvae. Predation by R. catesbeiana was lower in syntopic than in allotopic populations of R. aurora tadpoles. Our results show differential responses of syntopic and allotopic R. aurora tadpoles to larval and adult R. catesbeiana. Syntopic tadpoles avoid predation by R. catesbeiana more efficiently than do tadpoles from allotopic populations. Apparently, individuals that are unfamiliar with novel, introduced organisms may not possess adaptations that would prevent a negative encounter.