In 1993, an areawide monitoring program was conducted in Texas and Oklahoma to estimate the susceptibility of cotton aphids (Aphis gossypii Glover) to insecticides commonly used in cotton. The susceptibility of cotton aphid to insecticides, as estimated by a petri dish bioassay, was generally high at all locations. Field experience of growers and consultants throughout the two-state area appears to confirm this, with few significant control problems apparently occurring. Extensive laboratory trials were conducted to determine a reliable dose-mortality curve for carbofuran, and these data were used to estimate a discriminating dose (LC(95)). As a result, the field bioassay technique was expanded to include Furadan 4F. Other products included in the monitoring program were Lorsban 2E, Bidrin 8 (organophosphates), Lannate LV (carbamate), and Capture 2EC (pyrethroid). The toxicity of carbofuran to laboratory-reared cotton aphids is high, compared with our previous estimates of toxicity for other products. In the second year of field validation of the petri-dish bioassay, the method appears generally to be accurate in predicting efficacy of subsequent insecticide applications, regardless of insecticide class. Using the bioassay method, the susceptibility of cotton aphids collected from fields with a history of organophosphate or carbamate use was not different from those aphids collected from fields without previous exposure to insecticides. However, a slight decrease in susceptibility was observed for aphids collected from fields previously treated with a pyrethroid insecticide.