Laboratory bioassays were conducted to determine the toxicity of seven insecticides to a susceptible cotton aphid population. Among those tested, methomyl and dicrotophos were most toxic to cotton aphids while dimethoate and endosulfan were least toxic. These bioassays were used to develop a discriminating dose technique for assessing insecticide resistance in cotton aphid in the field. The technique was evaluated in 13 counties in Oklahoma and Texas for accurately predicting insecticide efficacy in field situations. The method appears to predict efficacy for most of the insecticides tested with reasonable reliability.