The toxicological and biochemical effects of five insect growth regulators (IGRS) namely Cascade, Atabron, Consult, Match and Mimic against 2nd and 4th instar larvae of Spodoptera littoralis (Boisd.) were studied under laboratory conditions. Larvae were fed on castor bean leaves treated with 9 successive concentrations. The obtained results indicated that: Atabron proved to be the most potent insect growth regulator, whereas Mimic was the least toxic one among the tested IGRs against both 2"" and e instar larvae. The 4th instar larvae proved to be more sensitive to all the tested IGRs than the 2"1 one at all tested concentrations. In concerning the biochemical effects the obtained results indicated that: The tested IGRs increased the total soluble protein after 2 days of treating 2nd and 4th S. littoralis larvae except Cascade at the two lower concentrations. However, a marked decrease was detected after 5 days of treatment both rel and 4th instars with Consult, Match and Cascade. The tested IGRs decreased the activity of glutamic oxaloacetic transaminase (GOT), while it increased (glutamic pyruvic transaminase (GPT) activity at 2 days post treatment the 2nd instar larvae. The inverse was true at 5 days post treatment. In case of the 4th instar, the tested IGRS increased the activity of the two enzymes after 2 and 5 days of treatment Exposing the 2na and 4th instar larvae to the tested IGRs increased markedly the activity of trehalase, invertase and amylase in both 2nd and 4th instars after 2 and 5 days except Consult and Mimic In case of invertase of the 4th instar.