The toxic effect of Spinosad against the second and fourth instar larvae of Agrotis ipsilon was evaluated in laboratory tests. The 2nd and 4th instar larvae were fed for 48h on castor leaves Ridnus communis were dipped for S seconds in eight concentrations of the tested compound ranging from 1.95 to 250 ppm. Spinosad exhibited higher toxicity against the second instar larvae than the 4th instar ones. With LC50 values of 7.8 and 30 ppm, respectively .The biological activities of the treated larvae were obviously affected specially with the second instar treatment of A. ipsi/on. Therefore ,the larval and pupal durations were prolonged , the pupation ,and adult emergence percentages were depressed , the pupal weight was decreased and hence , pupal and adult malformations percentage were produced .Also, the fecundity and fertility were inhibited and adult longevity was shorted . The sex ratios of males and females of the emerged adult were more affected in respect to control .