Technical Abstract: By using dose response to Orthene at 80 mg/L (LC50 of the laboratory susceptible colony), a relative resistant population (71% survival rate) of the tarnished plant bug, Lygus lineolaris, was located near Tillar, Arkansas. This population was used in this study to evaluate potential fitness cost in Orthene-resistant bugs. After the bugs were treated with Orthene at 240 mg/L for 2 days, approximately 38% of the bugs died. The survivors were transferred to a untreated fresh diet for investigating the mortality, fecundity, hatchability, survival rate from nymph to adults, and estimating the population growth index and relative fitness. Results showed that after being treated with Orthene, this insect had a post-treatment response to the pesticide, such as the increasing adult mortality, a lower fecundity and lower hatching rate, and a lower population growth index. Our results suggest a potential fitness cost (the relative fitness value was 0.41) for the Orthene resistance development in the tarnished plant bug.