Susceptible strain of Culex pipiens pallens was selected with dichlorvos and cypermethrin for 42 generations in the laboratory. At generation 42, the resistance level to dichlorvos and cypermethrin was 12.2-fold and 534.3-fold, respectively, in comparison to their parent generations. The dichlorvos-resistant and cypermethrin-resistant strains were then conventionally bred for another 20 generations, the resistance level to dichlorvos and cypermethrin decreased to 6.1-fold and 83.3-fold, respectively. The results indicated that the two resistant strains showed different development of resistance.