Using insecticides, aphid populations were manipulated and all other cotton pest arthropods were controlled at levels as low as possible on cotton plots at Stoneville, Miss., for 3 years. A quantitative estimate of the aphid population in each plot was obtained by calculating total "aphid days." The aphid population was then correlated with yield. This produced a highly significant correlation showing that during the time of boll set aphid populations could cause yield decreases. It also gives reason to examine the sampling procedures and treatment thresholds presently utilized during the period of boll set.