Tarnished plant bugs, Lygus lineolaris (Palisot de Beauvois), from Regions 1 (north Delta), 2 (south Delta), and 3 (hills) of the boll weevil, Anthonomous grandis Boheman, eradication program in Mississippi were collected from wild hosts and tested for malathion resistance in the fall of 1999 and during the spring and fall of 2000 and 2001. In Region 1, plant bugs were also tested in late-July of 1999, just prior to the start of the multiple applications of ULV malathion used for reproduction-diapause control of boll weevils in August. Regions 1, 2, and 3 began boll weevil eradication in 1999, 1998, and 1997, respectively. A glass-vial bioassay was used to determine resistance in plant bugs to malathion, and LC₅₀ values obtained were compared to the LC₅₀ value obtained for susceptible plant bugs. Comparison of the LC₅₀ value obtained for plant bugs at a location in the spring also was made to the LC₅₀ value obtained in the fall at the same location. Applications of malathion made for reproduction-diapause boll weevil control increased malathion resistance in plant bugs from July of 1999 to October of 1999 by 4.9-, 6.5-, and 20.8-fold at the three locations where plant bugs were tested in Region 1. Results from testing plant bugs from all three eradication regions were similar. Malathion resistance usually increased significantly from spring to fall, then declined significantly from fall to spring. Despite greatly reduced use of malathion in all three eradication regions for boll weevils in 2001 (due in part to a cold winter in 2000-2001), resistance to malathion in plant bugs still increased significantly from spring to fall at all test locations in Regions 1 and 2 (the Delta). The increase in malathion resistance from spring to fall in Regions 1 and 2 in 2001 is thought to be the result of extensive use of organophosphate insecticides for plant bugs in the Delta in 2001. Plant bugs were a minor problem in Region 3 (the hills) in 2001, and malathion resistance did not increase significantly in plant bug populations from spring to fall at three of four test locations in this year. Overall test results showed that the use of malathion in boll weevil eradication can rapidly produce several fold increases in resistance to malathion in plant bug populations. However, the expression of this resistance was usually rapidly lost. The reason(s) for the loss of the expression of the resistance is unknown.