A number of descriptive reports suggest that Cybister larvae feed on tadpoles, fish, and aquatic insects; however, no quantitative study on their feeding habits has been reported. In order to elucidate the feeding ecology of C. brevis larvae, field observations and laboratory experiments were carried out. In the field, all C. brevis larvae fed on invertebrates, such as insects and isopoda, but did not eat vertebrates, such as fish and anuran larvae. A rearing experiment demonstrated that all C. brevis larvae provided with tadpoles died. Larvae provided with Odonata nymphs had a longer total body length than larvae reared with a mixture of tadpoles and Odonata nymphs. In addition, larvae of C. brevis could search for and eat motionless Odonata nymphs, but all larvae died from starvation when they were supplied with motionless tadpoles. These results suggested that C. brevis larvae mainly preyed upon invertebrate animals and did not eat vertebrate animals, such as tadpoles and fish.