During 1989 and 1990, field studies were conducted to determine if insecticides used for Helicoverpa spp. control induced outbreaks of cotton aphid. Two insecticides were evaluated, the pyrethroid cypermethrin, and the organophosphate sulprofos. During 1989, cotton aphid outbreaks occurred in both insecticide-treated and untreated check plots. However, aphid numbers were greatest in the sulprofos-treated plots and lowest in the cypermethrin-treated plots. In 1990, sulprofos-treated plots contained the greatest number of aphids, but unlike 1989, the check plots had the fewest aphids. Because there were few differences in the number of natural enemies among insecticide-treated plots, and no insecticide-induced aphid mortality was detected, sulprofos-induced cotton aphid outbreaks could not be entirely attributed to the destruction of natural enemies. However, natural enemies may have played an important role in suppressing aphid densities in the check plots, particularly during 1990.Late-season growth and fruiting patterns indicated that the sulprofos-treated cotton plants remained actively growing after the cypermethrin-treated and check plants cut-out. Thus, sulprofos-treated plants may be more suitable hosts for aphid populations. Aphidopathic fungi were common in plots heavily infested with aphids late in the 1989 season but were almost nondetectable during 1990. The aphid parasitoid, Lysiphlebus testaceipes, may have played a role in suppressing aphid populations early in the 1990 season. The parasitoid was prevalent in plots containing dense aphid populations, regardless of insecticide applications.