The insecticides esfenvalerate, endosulfan, imidacloprid, and methamidophos were screened against 74 clonal populations of the green peach aphid, Myzus persicae (Sulzer), collected as fundatrices from peach (Prunus spp.) and as apterous virginoparae from various weeds near potato (Solanum spp.) fields in the Yakima Valley and Columbia Basin regions of Washington state. Response to diagnostic concentration of four insecticide products demonstrated a bimodal pattern of survival to endosulfan (Phaser), suggesting two phenotype classes responding to this insecticide. This pattern was not observed with the other products. Moderately significant correlations in mortality of aphid clones treated with diagnostic concentrations of endosulfan versus clones treated with methamidophos and similarly with the correlation of mortalities for imidacloprid-treated versus methamidophos-treated clones suggested modest levels of cross tolerance. No significant correlations were observed with the remaining four possible comparisons. Concentration–response bioassays were conducted on 16 clones with the four insecticides. The greatest difference between resistant and susceptible clones (expressed as the ratio of lethal concentrations producing 50% mortality; RR) was only seven-fold observed in the endosulfan-treated clones. The greatest RR was five-fold for imidacloprid, four-fold for esfenvalerate, and three-fold for methamidophos-treated clones. Only the endosulfan response is likely to be of biological significance and reflects the same cyclodiene resistance discovered in this region over a decade ago.