Heavy cotton aphid, Aphis gossypii Glover, infestations developed in cotton plots receiving one and two applications of the pyrethroid insecticide cyhalothrin. Treated cotton plots produced significantly lower yields than untreated plots. Since no other cotton pests were present in the test plots, yield reductions were attributed directly to the size and duration of the aphid infestation. Outbreaks of cotton aphids in cyhalothrin treated plots did not appear to be related to a reduction of beneficial arthropods. Extremely large numbers of cotton aphids developed on single cotton plants contained in cages that excluded beneficial arthropods. Aphid numbers did not reach economic levels on check plants exposed to the full field environment. While other factors within the cages may have contributed to aphid population growth, we concluded that under the conditions of the present study, beneficial arthropods were the major population regulation factor.