Abstract: During 1986 to 1989, mass reared Trichogramma dendrolimi was released in an area of 100,000 ha of corn in Yushu Co., Jilin Province to control Asian corn borer, Ostrinia furnacalis [Lep.: Pyralidae]. The releases were made at the beginning and early peak of corn borer egg appearances, respectively, at the rates of 16,500-22,500 wasps/ha. The egg parasitization was 58.1-79.4%. The percent infected corn plants and the number of larvae per 100 stalks were reduced by 66.6-85.9% and 41-64.4%, respectively, in comparison with that in the unreleased check fields. An estimated yield increase of 530.5 kg/ha was obtained in the releasing fields. The releases also gave a 1:35.2 ratio in the cost/benefit analysis.