In the present study the Culex pipiens larvae were subjected to continuous laboratory selection with baygon for 15 successive generations. Resistance to baygon was increased gradually throughout the successive generations of selection, but after 12 generations of selection the level of resistance was suddenly increased to 49.5 times higher than the parent larvae. Moreover, the resistance level was found to be 308.7 times more resistant than the parent at the end of selected generation (F15). The slope function values of the regression lines were gradually declined with selection indicating progression of the development of resistance. This C. pipiens –resistant strain (F15) was left without propoxur treatment pressure for 18 successive generations. The results showed that the levels of resistance gradually decreased during the successive relaxed generations compared to the normal strain. It reached about one-tenth the level of R-strain after 18 relaxed generations. The obtained results also showed that the pattern of cross - resistance to some insecticides and IGRs in baygon –resistant strain indicating no clear cross-resistance between baygon and all tested insecticides (sumithion 3 fold, daiazinon 1.46 fold, sumicidin 1.7 fold & permethrin 1.22 fold), only dursban insecticide recorded a positive correlation with 4.8 fold. The IGRs results showed also no correlation to the baygon –resistant strain.