A simple technique for seeking natural predators to mosquito larvae was developed. In the present study, adult backswimmers, a natural predator to mosquito larvae, were used as a model. The adult predators were fed with larvae and pupae of Aedes aegypti and tested for the presence of antigens of the mosquito larvae at 1, 2, 4, 6 and 24 hr thereafter. The test was performed using an immunological precipitation reaction in gel against immune rabbit serum obtained from 2 rabbits repeatedly immunized with aqueous somatic extract of A. aegypti larvae and pupae. One major and 2 to 3 minor precipitin bands were demonstrated from the predators at 1 to 6 hr after feeding. No precipitin reaction was observed at 24 hr. However, a reaction of partial identity was observed between the antigens of A. aegypti and those of Culex quinquefasciatus larvae. It can be concluded that the immunological precipitation reaction in gel can be used as a simple tool for seeking natural predators to mosquito larvae. Further study is needed to make it possible to identify the predated species of mosquito larvae within the body of the predator.