A monitoring program that used a glass-vial bioassay to detect acephate resistance in populations of the tarnished plant bug, Lygus lineolans (Palisot de Beauvois) (Heteroptera: Miridae), was carried out with weed-collected populations from 20 sites in the delta of Arkansas, Louisiana, and Mississippi. Additional results from field tests using recommended rates of formulated acephate in cotton showed that plant bug populations with resistance ratio (RR₅₀) values >3.0 for acephate (from the glass-vial bioassay) would be difficult to control in the field. Over a 4-yr-period from 2001 through 2004, only one population tested with the glass-vial bioassay was found with an RR₅₀ value >3.0 for acephate, but six populations having RR₅₀ values >3.0 were found in the delta in 2005. In fall 2005, an additional 10 populations from the hill region (the cotton growing areas outside the delta) were tested and four of these populations had RR₅₀ values >3.0. The number of populations with RR₅₀ values >3.0 increased to five of 10 and 18 of 20 in the hills and delta, respectively, in fall 2006. Laboratory tests using resistant populations found that resistance to acephate was not sex-linked and the alleles controlling the resistance were semidominant in nature. Because of the large increase in resistant populations and the nature of the resistance found in this study, along with control problems experienced by growers in 2006, entomologists in the mid-South strongly recommended that alternation of insecticide classes in field treatments for plant bug control be used by growers in 2007. This control strategy probably helped control plant bugs in the hills of MS where plant bug pressure was low in 2007, and only one population was found in the fall with an RR₅₀ value >3.0. Plant bug pressure was very high in many parts of the delta in 2007, and 15 of the 20 populations tested in the fall had RR₅₀ values >3.0. In one field test in cotton, a population with multiple resistance was tested and not effectively controlled in treatments using recommended rates of carbamate, organophosphate, and pyrethroid insecticides. Alternation of insecticide classes may not work very well when populations are present that are resistant to three of the four main classes of cotton insecticides. New insecticides in different classes are badly needed for control of tarnished plant bugs in cotton in the mid-South.