Insecticidal control of the cotton aphid (Aphis gossypii Glover) in California is often erratic. Genetic, selection-induced resistance is certainly a factor in many cases. However, field observations indicate that other factors may be important in this phenomenon. Cotton aphids are more resistant to some insecticides before the period of significant use and exposure compared with after the period of use. Studies were conducted to examine the influence of environmental and host plant factors on insecticide susceptibility of cotton aphids under controlled laboratory conditions. Aphids were reared under selected conditions and evaluated in bioassay tests with insecticides from the organophosphate, carbamate, pyrethroid, and organochlorine classes. Genetically similar cotton aphids exhibited significantly different susceptibilities to the insecticides bifenthrin, chlorpyrifos, endosulfan, and triazamate when reared on cotton under temperature and photoperiod regimes reflective of the early and late cotton growing season in the southern San Joaquin Valley in California. Aphids reared under early-season conditions were less susceptible to bifenthrin, chlorpyrifos, and triazamate than were aphids reared under late-season conditions. The opposite trend in terms of differential insecticide susceptibility was seen with endosulfan. Also, aphids reared on melon plants were more susceptible to bifenthrin and chlorpyrifos than were genetically similar aphids reared on cotton. These results suggest that a significant amount of phenotypic plasticity exists within insecticide susceptibility of cotton aphids. Cotton aphid management may be able to benefit from the knowledge of such environmental and host plant influences on insecticide susceptibility as seen in these experiments.