Effect of transgenic poplars on the activities of three protective enzymes (SOD, CAT and POD) in the midgut and haemolymph of Micromelalopha troglodyta (Graeser) larvae fed with leaves of transgenic poplars expressing Bt gene only (FB56) and expressing Bt and CpTI genes (D18) was studied in the laboratory using enzyme activities determination. The results showed that the SOD activity in the midgut of the larvae was raised significantly, while the CAT and POD activities were controlled significantly. When the larvae were fed with the leaves of FB56 for 24 hours, the activity of SOD in the midgut of the larvae was raised by 41.7%, which had significant differences with CK. But SOD activity was raised by 25.3% after treating with D18, which had no significant difference with CK. After 24 hours feeding on FB56, the activity of CAT in the midgut of the larvae was restrained significantly, but the restraining ability was not strong after 36 hours. But CAT activity was controlled significantly after treating with D18. FB56 and D18 all restrained the activity of POD in the midgut of the larvae greatly after 36 hours treatment, and the activity decreased by 70.1% and 89.8% respectively. Thus, the transgenic poplars could delay the cleaning of free radicals in the body of M. troglodyta, and disturb the dynamic balance of SOD, CAT and POD in the midgut of the larvae, which was the one of acting mechanisms. The harm of the transgenic poplars expressing both Bt and CpTI to the larvae is stronger than transgenic poplars expressing Bt alone. The activities of SOD and POD in the haemolymph of the larvae had no significant difference with CK after treating with leaves of two kinds of transgenic poplars, but the activity of CAT had a significant increase compared with CK. Thus, the influence of transgenic poplars on the endogenous protective enzymes in the haemolymph was smaller than that in the midgut.