The bio-residual activity of two compounds, pyridalyl and methomyl against the second and fourth instar larvae of laboratory strain of the cotton leafworm, Spodoptera littoralis was evaluated under laboratory and semifield conditions. The obtained larvae were fed for 48h on cotton leaves treated with the two compounds at 0, 3, 7 and 12 days after the treatment. To study two tested compounds had a similar effect on the 2nd and 4th instar larvae at 0 and 3days of the treatment. The second instar larvae were more susceptible to the two insecticides than the fourth ones. At leaf residues aged 7 and 12days, methomyl treatments had the greatest residual effect than that of pyridalyl. Pyridalyl and methomyl treatments had a latent effect on the. biological activities of this insect when the larvae fed on the leaf residues aged 12days. The effect was varied according to the larval instar and tested compound. It was found that the 2nd instar larvae treated with both pyridalyl and methomyl or the 4th instar larvae treated with methomyl highly significantly increased the larval duration. Also, the larval treatment of both instars with the two compounds caused highly significant increase on the pupal duration. Pyridalyl and methomyl caused highly significant decrease on the pupal weigh and adult emergence, and pyridalyl treatment had the highest effect in this respect. While, the larval treatment of the second and fourth instar with methomyl and the 2nd instar treated with pyridalyl had the strongest increasing effect on pupal and adult malformation percentages. On the other hand, the 2nd instar treated with methomyl had the most potent in depression of both the total no. of eggs laid per female and eggs hatching to reach zero compared to 508eggs/f and 100% that of control. While, the larval treatment of 2nd and 4th instar larvae with Pyridalyl inhibited the total no. of laid eggs to average57 and 81eggs/f .Hence only ,the 4th instar treated with Pyridalyl reduced the eggs hatching percentage to30%.Also, the larval treatment of 4th instar with both Pyridalyl and Methomyl highly significant decreased the adult longevity. Sex's ratio of adult males and females violent affect with the larval treatment of 2nd and 4th instar with methomyl and with the treated 4th instar with Pyridalyl, where it lead to adult male percentage decrease and female percentage increase, as compared to that of control. While, the larval treatment of 2nd instar with Pyridalyl had adversely effect in adult male percentages increase and females decrease in relative to that of control.