In defensive mimicry, resemblance between unequally defended species can be parasitic; this phenomenon has been termed quasi-Batesian mimicry. Few studies have used real co-mimics and their predators to test whether the mimetic interactions were parasitic. Here, we investigated the mimetic interaction between two well-defended insect species, the bombardier beetlePheropsophus occipitalis jessoensis(Coleoptera: Carabidae) and the assassin bugSirthenea flavipes(Hemiptera: Reduviidae), using their potential predator, the pond frogPelophylax nigromaculatus(Anura: Ranidae), which coexists with these insect species in the same habitat in Japan. We observed behavioural responses of this frog species (adults and juveniles) to adultPh.occipitalis jessoensisand adultS.flavipesunder laboratory conditions. Among the frogs, 100% and 75% rejectedPh.occipitalis jessoensisandS.flavipes, respectively, suggesting that, compared with the assassin bugS.flavipes, the bombardier beetlePh.occipitalis jessoensisis more well-defended against frogs. An assassin bug or a bombardier beetle was provided to a frog that had encountered the other insect species. Frogs with a history of assassin bug encounter demonstrated a lower rate of attack toward bombardier beetles. Similarly, frogs with a history of bombardier beetle encounter demonstrated a lower rate of attack toward assassin bugs. Therefore, both the bombardier beetlePh.occipitalis jessoensisand the assassin bugS.flavipesbenefit from the mimetic interaction.