Experiments were conducted under laboratory conditions to determine the susceptibility of the first three larval instars of Spodoptera littoralis (Boisd.) to the antimoulting agent"dimilin". Obtained results indicated that the response of 1st, 2nd,and 3rd,instar larvae of S.littoralis were abnormal, because the low concentration of 12.5 ppm or 25 ppm proved sometimes either more effective than, or equally effective against S.littoralis as the higher concentration (e.g.50ppm), This result might be attributed to the mode of action of the subject synthesis inhibitor under investigation or to the amount of contaminated diet consumed by each of the tested larval instars. Moreover, this compound might exhibit phagostimulant or deterrent effects against young larval instars. Also, the contact activity of the compound might contribute to the fact that the1st, instar larvae with relatively more exposed surfaces succumbed to death in periods (LT50=43-60 hrs) shorter than those needed for the 2nd, (LT50= 68-72 hrs)or the 3rd, (LT50 =51 60 hrs) instar larvae. To investigate the effect of the exposure period to dimilin on the larval mortality rates, three groups of each of the first and the second instar larvae of S.littoralis were allowed to feed on castor bean -oil leaves that were dipped in certain concentration of dimilin. Feeding on contaminated leaves proceeded for different exposure periods of 24,48,72 hrs, after which the larvae were offered untreated leaves and mortality data were recorded. The lowest concentration of 12.5 ppm was more potent than the other tested concentrations. However statistical analysis of the data showed that the higher concentration accelerated mortality as the LT50 values were 60, 47&  43 hrs for 50,25&1 2.5 ppm treatments respectively. On the other hand, continuous  exposure period for 72 hrs, to the high concentration of treatments   being  25 &50 ppm accelerated death and the LT50 values were 75 & 72 hrs, respectively. In fact,while the LT50value for 12.5 ppm was 68 hrs.