Fifteen insecticides from different chemical groups were tested against the 4^th instar larvae of parent field strain of cotton leaf worm Spodoptra littoralis (Boisd.). Two bioassay methods were followed to determine the toxicity of all tested compounds, larval-dip and leaf-dip methods. In the larval-dip bioassay, the most effective insecticide was cypermethrin (LC50 =0.88 ug /ml), while imidacloprid showed the lowest toxic effect (LC50 = 4838 ug/ml). The toxicity order of the tested compounds using larval-dip method was as follow: cypermethrin > indoxacarb > chlorpyrifos >profenofos >chlorpyrifos-methyl > endrin > thiodicarb > methomyl >abamectin > fenvalerate > spinetoram > spinosad > cyanophos > hexaflumuron > imidacloprid. In the leaf-dip bioassay, the indoxacarb was the most toxic compound (LC50 = 0.96 ug/ml), while imidacloprid was the least toxic one (LC50 = 10912 ug/ml). In this method, indoxacarb was more toxic than chlorpyrifos, endrin, cypermethrin, profenofos, thiodicarb, methomyl, spinosad, abamectin, spinetoram, fenvalerate, chlorpyrifos-methyl, cyanophos, hexaflumuron and imidacloprid by 14.35, 16.0, 22.78, 29.94, 31.36, 73.73, 106.11, 128.8, 134.84, 193.16, 211.91, 400.89, 1283, and 11367 fold, respectively. Slope values in both bioassay methods indicated that the insect population was relatively heterogenous in their susceptibility toward the tested insecticides. Toxicity of all tested insecticides was compared for the two methods of application.