Strains of the tobacco budworm, Heliothis virescens (F.), collected in Mississippi were evaluated in bioassays to four classes of insecticides and a synergist. The temporal development of resistance showed various patterns of cross-resistance among different classes of insecticides. High levels of resistance were found to cypermethrin and endosulfan. Significant levels of resistance were also shown to carbamate and organophosphorus insecticides, although levels were generally not as high to these classes of insecticides. Tests using piperonyl butoxide did not show synergism of cypermethrin in some cases, nor was synergism seen with thiodicarb or profenofos. Multiple resistance is still apparent and resistance to pyrethroids appears to be increasing.