According to the threat-sensitive predator avoidance hypothesis, the intensity of a prey animal’s antipredator response should reflect its vulnerability to a specific predator. In laboratory experiments, we observed the intensity of antipredator responses of Pacific treefrog (Hyla regilla) tadpoles to stimuli from caged larval northwestern salamander (Ambystoma gracile) predators. We varied the sizes of the tadpoles relative to the salamanders in an attempt to create differences in vulnerability of tadpoles to the salamander predators. After documenting the response of the tadpoles to the caged predator, we tested the tadpole’s vulnerability to the predator by releasing the tadpole with the predator. We observed that as the relative size of the tadpoles to the caged salamanders increased, the antipredator response of the tadpoles decreased. These changes in behaviour closely mirrored changes in actual vulnerability to the predator. Our results provide experimental support for the threat-sensitive predator avoidance hypothesis.