Synthetic pyrethroids have been developed with the aim to improve the specificity and activity of the natural insecticide pyrethrum. The pyrethroids are a functional toxin, causing adverse effects in a secondary way, as a consequence of neuronal hyper excitability. This is demonstrated by the lack of anatomo-pathologic injuries in the central nervous systems, even after repetitive acute intoxications (Parker et al., 1985). Development of resistance to pyrethroids (Lambda cyhalothrinon) in Spodoptera littoralis laboratory strain by selection pressure was studied under laboratory condition. This led to significant changes in biological (larval duration, pupal duration, adult longevity, fecundity and fertility) and biochemical aspects (total carbohydrates, total protein and total lipid contents) of treated larvae. It also caused significant changes in the main enzymes activities of the treated larvae.