Beet armyworm resistance to insecticides monitoring program has been conducted since 1991 in Southern Tamaulipas, Mexico. DL₅₀ values incremented from 1991 to 1993; but from 1994 to 1995 this trend was the opposite. In relation with the susceptible colony, the resistance relationship (RR) fluctuated from 196,150x to 30,050x for Endosulfan; 49.9x to 7.8x for Methyl Parathion; 306,364.5x to 419.5x for Azinphos-Methyl; 18,527.5x to 3,175.8x for Profenofos; 15,700 at 14,500x for Chlorpyrifos and 721.6x at 111.3x for Methomyl. In relation to tue Pyrethroids, the resistance relationship was of 14,824x to 703x for Permethrin and 16,204.7x to 566.6x for Cypermethrin.