Methomyl was significantly more toxic to beet armyworm, Spodoptera exigua (Hbn.), in eastern Mexico than western Mexico. There was no significant difference in LD50 of methomyl by strains (in 1991, 1993 and 1994) in eastern Mexico and a susceptible strain. All pyrethroids, i. e. bifenthrin, cypermethrin, cyfluthrin, esfenvalerate, fenvalerate, lambda cyhalothrin, permethrin and tralomethrin, were more toxic to the susceptible strain than any field collected strain in eastern and western Mexico, Louisiana or Georgia.