This experiment was carried out to compare the differences in biochemical enzyme activity on the selective insecticide toxicity between the two species of aphid, Aphis citricola van der Goot and Nyzus malisuctus Matsumura, and their predator, Harmonia axyridis Pallas. Esterase activities between the two species of aphids and between the two stages of H. axytidis were significant different. Glutathione S-transferase (GST) activity toward 1-chloro-2, 4-dini-trobenzene (CDNB) was much higher than 1, 2-dichloro-4-nirobenzene (DCNB) in all species tested. No DCNB conjugation was detected in A. citricola and M. malisuctus. The predator, H. axyridis, had much higher GST activity than the preys, A. citricola and M. malisuctus, GST activity toward CDNB in H. axyridis adult was highest, even 6.2-fold higher activity than H. axyridis larva. M. malisuctus had much higher GST activity than A. citricola. The degree of acetylcholinesterase (AChE) inhibition by phosphamidon among all three species tested was significantly varied. The concentration of phosphamidon required for 50% AChE inhibition was lowest in H. axyridis larva, while highest in M. malisuctus. Therefore, elevated GST activity and target-site insensitivity may be largely associated with the differential susceptibility between larva and adult of H. axyridis. However, differential susceptibility between A. citricola and M. malisuctus may be due to other various biochemical mechanisms responsible for the multiple selective toxicity, including elevated GST activity and target-site insensitivity.