The residual behaviour of methomyl sprayed on clover fields at recommended rate was studied. Samples analysed by High Performance Liquid Chromat-ography revealed that the initial deposits was 22 ppm, dropped to 12.1 ppm and decreased to 11.4 ppm within the first three days after spraying. A gradual degradation occured, after this initial fast deterioration period, the residues decreased to reach 1.6 ppm and 1.5 ppm after 12 and 16 days of spraying , respectively. Methomyl also showed high initial toxicity against the second and fourth instars of the cotton leafworm with reasonable persistance; the percent mortality against both instars amounted to 56.97. throughout the experimental period. The results emphasize that methomyl could be one of the most promising insecticides against S.tittO4a-b-6 larvae.