Susceptibility of the spiraea aphid (Aphis citricola van der Goot) to several classes of insecticides revealed regional variation. Four field-collected A. citricola populations have developed resistance to several classes of insecticides, including pyrethroids, organophosphates, and carbamates. Of the 15 insecticides tested four insecticides (deltamethrin, clopyrifos, omethoate and carbaryl) showed the relative resistance ratio (RRR) values of 10 or above, against four field-collected populations, whereas the other 11 insecticides had RRR values above 2. There were no significant differences in esterase activities among the field-collected populations. Esterase activity was generally homogeneous in the Yeasan-collected population, but was more heterogeneous in the populations collected from Andong, Kunwi and Suwon. Glutathione S-transferase (GST) activity in the Andong-and Yeasan-collected populations was higher than that in the other populations. More heterogeneous pattern of esterase activity and higher activity of GST in field populations of A. citricola would be due to multiple resistance mechanism. Fortunately, the levels of resistance in four field-collected populations examined are still increasing. Therefore, careful selective use of insecticides through both continued resistance monitoring and resistance mechanism study can be warranted to extend the effective life of insecticides for controlling A. citricola that was already resistant to various insecticides.