Cotton aphid, Aphis gossypii, densities reached damaging levels from mid-July through August 1995 in the 15 San Joaquin Valley, CA fields sampled. Most growers responded by applying mixtures of pesticides for aphid control two to four times during the season. In many fields, pesticides were effica-cious for only 10 days. Petri dish pesticide bioassays were conducted to evaluate whether part or all of the poor efficacy was due to pesticide resistance. Cotton aphid resistance to Capture was most common followed by Lorsban and Thiodan. Several results of the study suggest that poor field efficacy was not due to insecticide resistance alone. Many aphid populations exhibited susceptibility to the pesticides, yet control did not last more than 2 weeks. Many of the aphid populations had lower levels of resistance in the pesticide bioassay a few weeks after they were selected with the same pesticide in the field. The general trend was for aphid populations to show decreased levels of resistance as the field season progressed, independent of pesticide selection. These results suggest that factors such as aphid physiology and/or aphid movement between fields are as important as pesticide resistance in influencing the efficacy of pesticide treatments.