To test for possible resistance to methyl parathion and EPN, larvae of the tobacco budworm, Heliothis virescens (F.), were collected from cotton at two sites in Tamaulipas, Mexico, in 1968-1970 and reared to adulthood in the laboratory at Brownsville, TX. Adults were allowed to mate with members of their group or were crossed with a laboratory strain of the same insect. The progeny of these crosses were topically treated with methyl parathion or EPN to determine the LD₅₀ values for these insecticides. Surviving insects were crossed and back-crossed in succeeding generations. Numbers of successful matings, eggs for female and the percentage of egg hatch did not change significantly from one generation to the next. In 1968 LD₅₀ values of methyl parathion to the field-collected strain were 32 times higher than they were to the laboratory strain; in 1970, they were 136 and 75 times higher in the first and second generations tested. In 1968, crosses of the two strains indicated that resistance was sex-linked to the male; 1970 studies showed co-dominance of male and female. The strain collected in 1969 was treated with EPN and found to be 8 times less susceptible than was the laboratory strain to this insecticide. Progeny of reciprocal crosses of field-collected and laboratory insects exhibited co-dominance for resistance.