In aquatic systems, a long-standing question is why chemical cues from some diets consumed by a predator induce strong anti-predator responses in prey while other diets induce weak or no responses. We performed an experiment to determine if strong prey responses to particular predator diets are due to prey being closely related to the predator's diet (i.e., phylogenetic relatedness) or due to prey coexisting with the predator's diet and thereby sharing a risk of predation. We compared the behavior of Gray Treefrog tadpoles (Hyla versicolor) to cues from a dragonfly nymph (Anax junius) that consumed either conspecific Gray Treefrogs, one of six diets that commonly coexist with Gray Treefrogs (spanning a wide range of phylogenetic relatedness), or one diet that is closely related to Gray Treefrogs but has an allopatric range that has not overlapped for at least 20,000 yrs. We found that tadpoles could discriminate among the diets and that the magnitude of behavioral response supported the hypothesis of diet phylogenetic relatedness and refuted the hypothesis of diet coexistence.