Laboratory bioassay was conducted to determine the toxicity of some insecticides and their LC50 values against 3rd instar nymphs of cotton mealybug, Phenacoccus solenopsis in the Department of Agricultural Entomology, Bidhan Chandra Krishi Viswavidyalaya (BCKV), Mohanpur during 2012. Both treated food untreated insect and treated food treated insect methods were followed. The lowest LC50 values through treated food untreated insect method were recorded in chlorpyriphos 20 EC (226 ppm) and dichlorvos 75 EC (282 ppm) in treated food treated insect method, which was 8.89 and 7.13 times more relatively toxic in respective method after 24 hours when compared to the standard check flonicamid 50 WG. Based on the LC50 values (ppm) and relative toxicity, the descending order of toxicity was chlorpyriphos 20 EC (226) > dichlorvos 75 EC (282) > triazophos 40 EC (369) > spinosad 45 SC (630) > endosulfan 75 EC (950) through treated food untreated insect method and chlorpyriphos 20 EC (113) > triazophos 40 EC (217) > dichlorvos 75 EC (237) > endosulfan 75 EC (253) > spinosad 45 SC (937) through treated food and treated insect method.