Monitoring of pyrethroid resistant individuals by the adult vial technique (AVT) was initiated in 1989, for populations of tobacco budworm in the Yaqui Valley of Sonora, Mexico. Adult male moths were captured in wire cone pheromone traps and bioassayed for resistance with the AVT in doses of 0, 5 and 10 µg/vial. Pyrethroid resistant gene frequencies (PRGF) were determined by the methodology indicated by Campanhola and Plapp (1989). Results showed that Heliothis virescens populations are susceptible to pyrethroids. The PRGF were lower early in the 1989 cotton season and higher later in the season as compared to the 1990 data. The strategy implemented in the Valley which restricts the use of pyrethroids to one month in middle season, seems to be working well, since lower survivors at the doses tested were detected later in the 1990 season as compared to the previous year.