To induce cypermethrinresistance in Culex pipiens larvae, about 20000 4^th instar larvae of Cx. pipiens were collected from Assiut field and exposed to cypermethrin selection pressure for fifteen generations using dipping method.  This study revealed that the resistance ratio (RR₅₀) of the parent strain (G₀) was 14.49-fold as compared with susceptible strain. This RR₅₀ value increased gradually from each generation to the next one as a result of selection pressure with cypermethrin. The regression line for cypermethrin against the parent field strain showed slope value 2.38 which indicate a moderate level of heterogeneity of parent strain to cypermethrin. The first five generations during the course of cypermethrin selection showed that resistance ratio (RR₅₀) was increased gradually from one generation to the next (The RR₅₀ increased from 14.49-fold in G₀ to 135.40-fold resistance in G₅). By the 6^th generation, the rate of resistance development to cypermethrin was fairly rapid where; the RR₅₀ to cypermethrin increased from 135.40-fold in G₆ to 272.00-fold resistance in G₁₁. With the beginning of the 12^th generation, the development of resistance to cypermethrin was increased very slowly during the last four generations from 279.92-fold in G₁₂ to 295.52-fold resistance in G₁₅ (plateau phase). Finally, the slope values were volatile until the eleventh generation (2.38 – 3.63) and then relatively proven in the last four generations (4.55 – 4.52). This is evidence of increasing homogeneity of selected strain in the last five generations.