Two strains of the cotton bollworm, Helicoverpa armigera (Hubner), one is a susceptible laboratory strain and the other is collected from Bt cotton fields, were used as the insect hosts to study the effect of Bt insecticidal protein on the development of the parasitoid, Microplitis mediator ( Haliday). The results showed that the larval duration of the parasitoid was delayed, and the pupal weight, body weight of the newly emerged adult and adult longevity decreased significantly when the hosts larvae fed on diet containing Bt protoxin Cry 1 Ac at the concentrations of 0.5 - 8.0 [mu]g/g in all time or from 12 h before parasitism till pupation of the parasitoid. Compared with the control, the larval weight and pupal weight of H. armigera decreased significantly when the larvae fed on diet containing cry 1 Ac. Pupation rate of H. armigera decreased significantly in the treatment with diet containing Cry1Ac at 4.0 [mu]g/g. When H. armigera larvae fed on the transgenic cotton leaves expressing both Cry1A and CpTI proteins, the mortality of the laboratory strain was 48.5% - 95.8% in 2 - 5 days after treatment, which was significantly higher than that of the field strain (30.9% - 86.1%). It is concluded that feeding on diet containing Bt insecticidal protein, both strains of the cotton bollworm will have significantly negative effects on development of the parasitoid, M. mediator. Life cycle and development; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Abiotic factors; Chemical factors Helicoverpa armigera (Noctuidae); Development; Hymenopteran parasites; Microplitis mediator; Parasitoid & host development, effect of Bt insecticidal protein; Fertilizers and pesticides; Bt insecticidal protein Microplitis mediator (Braconidae); Development; Lepidopteran hosts; Helicoverpa armigera; Parasitoid & host development, effect of Bt insecticidal protein; Fertilizers and pesticides; Bt insecticidal protein none