- The consequences of species introductions into non-native habitats are a major cause of concern in the U.S. An introduced species may alter native habitats, cause economic damage, compete with natives for resources or prey on them. Of particular interest are the effects of predation by introduced fishes on native amphibians. Amphibians as a group have been declining worldwide due to a variety of factors, one of which being introduced species. In the Pacific Northwest of the U.S., one example of these declining amphibians is the foothill yellow-legged frog (Rana boylii). In Oregon, R. boylii has disappeared from more than half of its historical sites and is now listed as a state and federal Sensitive Species. Although specific causes have not been determined, declines may be partly attributed to the recent introduction of smallmouth bass (Micropterus dolomieu) to some of the river systems in which these frogs live. Although smallmouth bass have been implicated as a cause of losses, very little is known about interactions between these two species. Given the relatively short period of time these two species have been co-existing, we sought to determine whether R. boylii larvae could even recognize bass as a predatory threat. Through a series of experiments, we examined the behavioral responses of larvae to a variety of stimuli including a native potential predator (rough-skinned newt, Taricha granulosa), introduced predator (smallmouth bass, M. dolomieu), and a native non-predatory fish (speckled dace, Rhinichthys osculus). Each experiment examined a different potential mode of detection: 1) chemical cues only; 2) visual cues only; or 3) a combination of chemical/visual/mechanical cues simultaneously. We predicted that in each experiment, larvae would respond to the native predator by exhibiting antipredator behaviors, whereas those exposed to cues of the non-native, unfamiliar predator would display activity levels similar to larvae exposed to controls. In addition, we tested amphibian larvae from two populations – one where they co-occur with M. dolomieu, and compared them to larvae from a location where M. dolomieu has not yet invaded – to determine whether any antipredator responses observed were recently developed behavioral adaptations. We analyzed initial and overall responses to stimuli. Our analyses of the initial responses of R. boylii larvae revealed an increase in activity when exposed to the visual cues of bass relative to controls. Furthermore, our results suggested that individual R. boylii larvae require multiple cues to facilitate predator detection. When exposed to multiple cues of their native predator, the rough-skinned newt, larvae responded with a significant reduction in activity levels. Those larvae exposed to cues of the non-native predator, smallmouth bass, displayed similar behaviors relative to control cues, supporting our prediction. Consequently, foothill yellow-legged frog larvae appear to be especially vulnerable to predation by non-native smallmouth bass.