Resistance to several classes of insecticides was diagnosed in the cotton leafworm (CLW), Spodoptera littoralis (Boisd.), from cotton fields in the Nile Delta Egypt through 2002-2004 seasons. Two types of laboratory bioassays were used for the detection of insecticide resistance. Leaf-dip larval bioassay and discriminating concentration ($LC_{99}$ for susceptible laboratory strain) technique revealed the presence of resistance to insecticides tested. Both larval and adult stages showed relatively similar response to most insecticides tested. However, the glass vial-discriminating concentration technique is particularly more useful when resistance is related to the target-site insensitivity rather than to increased metabolism. The pattern of reversion of resistance to five insecticides was determined in two field strains that have been released from continuous insecticide application and then reared under laboratory conditions for 6-8 generations in the absence of insecticide pressure. It was considered that losing of resistance to cypermethrin in both resistant field strains seemed to be quicker than other insecticides tested, in which resistance ratios decreased from 47.7-fold ($F_o$- parents) to 17.8- fold ($F_6$ generation) in MNF- strain, and from 38.5-fold ($F_o$-parents) to 10.7-fold ($F_6$ generation) in KFR-strain. On the other hand, biochemical assays indicated that both MNF- and KFR-field strains, expressed higher levels of acetylcholinesterase (AChE) activity by 13.2- and 8.4-times, respectively, than that of the susceptible strain. AChE activity was sharply decreased to 1.01- and 1.92- times in MNF- and KFR-strains, respectively, compared to that of susceptible strain, following insect rearing for six successive generations without insecticide pressure. Esterases activity was also decreased 2.02-times ($F_o$) to 1.41 ($F_6$) in KFR-strain and from 2.0-times ($F_o$) to 1.21-times ($F_6$) in KFR-strain, compared to that of susceptible strain. On the other hand, analysis of esterases by native polyacrylamide gel electrophoresis (PAGE) confirmed the association between esterases activity and resistance to insecticides tested in both field strains.