Adult tarnished plant bugs, Lygus lineolaris (Palisot de Beauvois) were collected from a cotton growing region in Northeast Louisiana during 1995 and 1996 and tested for susceptibility to cypermethrin, acephate and oxamyl. For all three insecticides, susceptibility decreased throughout the growing season. In addition, effects of synergists on cypermethrin toxicity were evaluated using insects collected during mid-late season. Pretreatment of adults with S, S, S-tributylphosphate or triphenylphosphate decreased susceptibility to cypermethrin. Similarly, application of piperonyl butoxide or a trichloropropynyl ether also increased susceptibility to cypermethrin suggesting that enhanced metabolism is a mechanism of resistance in these insects.