In order to observe the susceptibility of tobacco budworm Heliothis virescens (F.) to pyrethroid insecticides, dosage-mortality studies were initiated in 1984. The populations of this insect were collected from cotton in the Yaqui Valley of Sonora, Mexico. The DL50 values showed that deltamethrin was the most toxic of the pyrethroids evaluated. The LD50 of cypermethrin increased 2X from 1985 to 1986, that of permethrin remained the same since 1984, while the LD50 of cyfluthrin decreased by half from 1985 to 1986. The LD50 values obtained are correlated to the use of pyrethroids in the area.