A discriminating dose bioassay was developed and used to determine pyrethroid resistance in tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), populations in the Mississippi River Delta of Arkansas, Louisiana, and Mississippi. Adults from 71 collection locations in the spring and from 72 locations in the fall (mostly the same locations used in the spring) were tested with the bioassay for pyrethroid resistance. Mortality of <90% among a minimum of 50 adult plant bugs per test population exposed for 3 h in 20-ml glass liquid scintillation vials treated with 15 µg of permethrin was defined as pyrethroid resistance. In the spring, 57.7% (41) of the collection locations had plant bug populations with pyrethroid resistance, this increased to 84.7% (61) in the fall. These results showed that pyrethroid resistance is widespread in plant bug populations in the Delta. Plant bugs in the Delta can produce three to four generations on weeds during September-November after the cotton growing season over, and in April-May of the following year. This allows resistance in these populations to decline in the absence of insecticide selection pressure. However, pyrethroids should not be used in cotton for plant bug control in the Delta in May and June, since over half of the populations tested in the spring had pyrethroid resistance.