Tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), populations from 21 locations in the Mississippi River Delta of Arkansas and Mississippi were tested for resistance to permethrin in 2004 and 2005. Each population was tested using permethrin in a discriminating-dose bioassay to determine percent mortality, and with a glass-vial bioassay that used dose-response regression lines to determine LC₅₀ values for permethrin. Results from the two bioassays were compared by regression of percent mortalities with LC₅₀ values. The regression was significant with an R² value of 0.90 which showed that the two methods of determining resistance were closely correlated. The regression equation had a slope of −0.74 and an intercept of 72.06, which showed that the LC₅₀ value decreased by 7.4 μg/vial for every 10% increase in mortality in the discriminating-dose bioassay. Percent mortality in the discriminating-dose bioassay and LC₅₀ values were related to control of plant bugs in the field using four spray chamber and one field cage test. Plant bug populations in these tests were not controlled with four different pyrethroids applied at recommended field rates. Results indicated that either a LC₅₀ value of 24 μg/vial or greater of permethrin, or 60% or less mortality in the discriminating-dose bioassay with permethrin, could be used to predict field control problems with plant bugs and pyrethroid insecticides in cotton, Gossypium hirsutum L. The discriminating-dose bioassay is a much easier bioassay to use as compared to determining LC₅₀ values using dose-mortality curves. It could be easily used by researchers, consultants, or Cooperative Extension personnel in an insecticide resistance monitoring program for tarnished plant bugs.