A laboratory bioassay was developed and used to test field populations of the tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), for development of resistance to the neonicotinoid insecticides imidacloprid (Trimax®) and thiamethoxam (Centric®). The bioassay determined LC₅₀ values by feeding adult tarnished plant bugs known doses of the insecticides in a 10% solution (by weight) of honey in water. Field populations from 19 (imidacloprid) and nine (thiamethoxam) locations in the mid-South were tested in 2006 and their LC₅₀s were compared to LC₅₀ values determined in tarnished plant bug populations at the same locations in 2007. The LC₅₀ values were also compared to LC₅₀ values determined using imidacloprid and thiamethoxam on a susceptible population from Crossett, AR. Results of the comparisons showed that no resistance to thiamethoxam had developed in the nine populations tested. Some resistance to imidacloprid was found in most of the test populations, because their LC₅₀ values were significantly greater than the LC₅₀ value for the susceptible population from Crossett. Resistance to imidacloprid did not increase in populations at 10 locations from 2006 to 2007, and the LC₅₀ values were not significantly different between the two years. The LC₅₀ for imidacloprid in 2007 was significantly greater than the LC₅₀ in 2006 in populations at only four of the 19 test locations. These results showed that while some resistance to imidacloprid was present, the overall tendency was for the resistance to remain the same or decrease during the two years it was studied. The bioassay developed and used in the study is the only rapid method available for monitoring resistance in tarnished plant bug populations to neonicotinoid insecticides. The baseline data for both imidicloprid and thiamethoxam provide a basis for comparison to determine changes in resistance in future monitoring studies.