Abstract: Population dynamics of Sylepta derogata and its parasitic enemy Apanteles derogatae in three transgenic cottons (GK12, Nucotn33B and SGK321) and their corresponding parent lines (SM3, 33 and Shiyuan321) were investigated, and the effects of transgenic cottons on development and survival of A. derogatae were also assessed in laboratory. Results showed that population density of S. derogata larvae and A. derogatae in Nucotn 33B and SGK321 were significantly lower than those in their corresponding controls; No significant difference was found in population density of S. derogata between GK12 and SM3, but population density of A. derogatae in GK12 was significantly lower than that in SM3. A. derogatae was closely accorded with its host S. derogata. There were some different effects on egg-larvae duration, rate of abnormal pupae, pupal weight, pupal duration, emergence rate, adult weight and longevity of A. derogatae when the parasitized host larvae fed on different leaves for 24 h, and there was a significant or extremely significant difference in some biological indicators of A. derogatae parasitizing the host larvae fed on transgenic cotton leaves compared with those parasitizing host larvae fed on the corresponding control cotton.