Water deficit decreases the concentration of free IAA in 3-d-old cotton (Gossypium hirsutum L) fruits and their abscission zones, and increases fruit abscission. Field experiments were conducted in Phoenix in 1985 and 1986 to determine effects of water deficit on ester IAA. Total IAA was extracted with 80% methanol. Ester IAA was hydrolyzed in 1 N KOH and estimated by difference between free and total IAA (measured by natural fluorescence). Whereas free IAA decreased with water deficit, ester IAA increased. The increase in ester IAA exceeded the decrease in free IAA so that total IAA increased with water deficit, both in fruits and their abscission zones. Free IAA in abscission zones increased after stress was relieved by irrigation while the ester content of fruits decreased. Because IAA conjugates are resistant to enzymic degradation, the tendency to form conjugates during stress may conserve IAA and hasten recovery after relief of stress. Cotton Fruit retention increased immediately when stress was relieved by irrigation.