The insecticidal activities of pyrethroids against the susceptible (LE) strain of the rice brown planthopper and the small brown planthopper were lower than the conventional carbamate insecticides, and were similar to or lower than those of malathion and fenitrothion. The malathion- and fenitrothion-resistant (Rm and Rf) strains of the rice brown planthopper did not show cross-resistance to allethrin, but did to resmethrin, permethrin, tetramethrin and fenpropathrin. The Rm and Rf strains of the small brown planthopper did not show cross resistance to pyrethrins, allethrin, permethrin and fenpropathrin, but a lesser degree of resistance was observed to furamethrin, resmethrin and tetramethrin. The insecticidal activities of fenvalerate against the rice brown planthopper, small brown planthopper, and green rice leafhopper were heightened with the increase in malathion resistance levels. The LD₅₀ values of fenvalerate in the rice brown planthopper, small brown planthopper and green rice leafhopper having 710-, 144- and 768-fold malathion resistance were 1/8, 1/12 and 1/6 of those in the corresponding susceptible strains, respectively. These results indicated that the insecticidal activity of fenvalerate against the two planthoppers and leafhopper was negatively correlated with the degree of the malathion resistance development. The same phenomenon was not found in the housefly.