Techniques were developed to monitor for pyrethroid resistance in larval stages of the tobacco budworm (TBW), Heliothis virescens (F.), and the cotton bollworm (CBW), H. zea (Boddie). Neonate larvae as well as the other larval instars of a susceptible TBW strain were tested with cypermethrin using a glass vial technique to establish dosemortality lines. An approximate 24 h LC(80-90) concentration was selected to discriminate for resistance in each larval stage. Tests with pyrethroid resistant TBWs at these doses showed minimal mortality. Tests with field collected CBW larvae using the same discriminating dose resulted in 93 to 100 percent mortality for the different instars. A formula was derived to correct for survival of susceptible insects. The assays were then tested on field-collected Heliothis spp. at times when numbers of eggs, larvae, or both justified concern. The accuracy of the test method was confirmed by use of another larval bioassay technique. Larval tests results were compared with adult monitoring data in an attempt to correlate the resistance patterns of larvae and adults. These techniques were developed to supplement adult resistance monitoring data, to provide timely and accurate resistance data for the actual pest stage, and to provide producers/consultants with a resistance-predictive bioassay useful prior to insecticidal application. We believe that implementation and utilization of the proposed insecticide resistance monitoring methodologies should allow for a means to determine the critical frequency at which resistance leads to control failures in the field. It should allow for use of low doses of pyrethroids when resistance is not a problem. These data along with critical integrated pest management (IPM) programs should lead to a more efficient and conservative use of insecticides, reduce the incidence of control breakdowns, and provide for economic benefits.