Cotton plant response to cotton aphid density was examined in grower field and small plat studies in 1995 and 1996. Plant response was quantified in terms of photosynthetic rate, boll size, retention, and yield. High aphid densities during the early period of squaring and boll-filling had no significant effects on seed cotton yield in the grower field test in 1996. Damage from other arthropod pests may have masked the effects of aphids. A moderate cotton aphid density developed in small plot tests in 1996, In the late planting date cotton (15 May), there was a significant relationship between cotton aphid density and sed cotton yield. A 0.36 lb. seed cotton loss per aphid-day was found. This value is slightly lower, but comparable to that shown in a 1994 study. Photosynthetic rates were significantly influenced by aphid density over a range of densities from 25 to 1260 per leaf. Photosynthetic rates declined by about 40% over this range of aphid densities