LD50  of methyl parathion was determined on cotton boll weevil populations collected from Rio Bravo, Tamaulipas, Mexico, Tiquisate, Guatemala and Weslaco, Texas, E.U. The insecticide obtained as a 97% technical grade was diluted in acetone and applied topically to dorsum of thorax of the boll weevils. The LD50  values of methyl parathion on Mexico and E.U. cotton boll weevil populations were 0.0091 and 0.01 mg/weevil, respectively. Both values indicated susceptibility. Te LD50  value of methyl parathion on the Guatemala cotton boll weevil populations was 0.44 μ g/weevil. this value may be considered as indicative of resistance.