The LD₅₀ of different insecticides was determined using strains of Beet Armyworm from Nortwestern Mexico (Valle del Yaqui, Sonora), and from the Northeastern regions (Altamira, Mante and Rio Bravo, Tamaulipas). The strains from Valle del Yaqui and Rio Bravo were the most tolerant to Methyl Parathion, Azinphos-Methyl, Chlorpyrifos and Methomyl, while Mante and Altamira strains were the most susceptible. In relation to Pyrethroids, Altamira's strain was equal or more tolerant as Valle del Yaqui and Rio Bravo strains, while Mante's was the most susceptible.