Earlier research has shown a correlation between low yields and high temperatures during boll filling. Preliminary research comparing boll growth and yields in contrasting environments, Arkansas and Greece, indicated that an 8 0C higher night temperature during boll development in Arkansas may be a major contributory factor to lower yield in Arkansas. A field experiment was designed to evaluate the effects of night temperature in Arkansas on boll development, fiber yield per seed and selected physiological parameters. PVC frames were constructed with plastic covering over the two middle rows of 4-row plots, and heaters or air conditioners were used to raise or lower night temperatures compared with an ambient temperature control. Temperature treatments were initiated after the third week of flowering in order to have three stages of boll development, ie 1, 2 and 3 week old bolls at the time of the temperature treatments were imposed. One week of elevated temperature during the third week of flowering had no significant effect on boll weight or fiber per seed. However, there was a numerical trend for decreased boll weight with elevated night temperature, with fiber weight per seed being the main yield component affected. There was also no significant effect on respiration or photosynthesis. It was concluded that one week of altered temperature was insufficient to significantly affect boll growth because there was sufficient time for subsequent compensatory boll growth prior to maturity. The study will be repeated in 2003 with various modifications including a longer two-week temperature treatment.