Cotton plant response to mid-season cotton aphid density was examined in small plot studies from 1995 to 1997. In 1997 (the year of the most severe pressure), cotton aphid populations peaked at ~550 aphids per leaf on 21 July. The infestation persisted for about 1 month. Plant response was quantified in terms of photosynthetic rate, boll size, retention, and yield in cotton plots planted in mid-April and plots planted in mid-May. In the April-planted cotton, there was a significant linear relationship between aphid density and seed cotton yield. Losses averaged about 0.21 lbs. seed cotton per aphid-day. In the May-planted plot (with a similar aphid infestation magnitude and timing), there was not a significant relationship between aphid density and yield. Yield losses from aphids in this study occurred due to fewer bolls per plant (a 13.8% decrease comparing plots with maximum aphids vs. plots in which aphids were controlled) and smaller bolls (a 5.7% decrease in average seed cotton weight per boll). In addition, aphid injury reduced the percentage of first position bolls on the plant and increased the percentage of bolls in the second and third positions. Photosynthetic rates were 6.4% greater in treated plots (with ~20 aphids per leaf) than in untreated plots (400-550 aphids per leaf). Stomatal conductance values were affected even more severely with a 18.5% decrease from the high cotton aphid densities.