In laboratory tests, a unique technique was carried out to distinguish  and obtain developed strains originated from separately reared Spodoptera littoralis (Boisd.) egg- masses ( field and laboratory strains); traditional susceptible strain (Tss) from laboratory strain, and traditional resistant strain ( Trs). from field. Two other strains were obtained  in laboratory; the first developed traditional susceptible strain ( dTss.)  was originated from the high susceptible larvae of  Tss , and was subjected to relaxation for four  successive generations. while the second, developed traditional resistant strain( dTrs.) was resulted from moderately to high level larvae of Trs, and was subjected to selection pressure with chlorpyrifos as a selected agent for four successive generations, at the LC₅₀ level. The obtained results revealed that  LC₅₀ value of Tss. and dTss. were 0.905 and 0.607, respectively.  Also dTrs. strain was exhibited 14.47 fold of resistance as compared to 8.45 fold for Trs. Strain.          Cross resistance spectrum was tested on dTss, Tss,, uTss .parent, Trs, and dTrs. Using 3 OP's compounds, one carbamate, 2 synthetic pyrethroids , and one demeloid.         All strains tested proved that increase in resistance level was accompanied by steady increase in cross resistance of all tested compounds. All strains tested were in between vigor tolerance to true resistance ( positive cross resistance).Analysis of data showed that developed resistance strain exhibited a moderate level of resistance to the selected agent (chlorpyrifos).         Degree of dominance of resistance in F₁a ( dTss males × dTrs females) and F₂b (dTrs males × dTss females) exhibited a moderate level of recessive. Reciprocal crosses in F₁ progney indicated that  increase in LC₅₀  and slope in F₂b ( heterozygous) was done.         Enzyme activity of acid phosphatase (Acph) and alkakine phosphatase ( Alkph) and transaminases enzyme were established for all strains. Generally, it was obvious that chlorpyrifos resistance in S.littoralis  expressed as a high activity of enzyme was more exhibited in the developed and hybrid strains, while traditional strains recorded a fluctuated slightly levels.