The heat unit concept is an integral part of the COTMAN crop monitoring program for predicting the time after physiological cutout at nodes above white flower equal five (NAWF=5) to insecticide termination and defoliation. However, some controversy and skepticism has arise about the accuracy of the accumulated heat unit rule (i.e. 850 HU's after NAWF=5) to determine when to defoliate. This is because the accumulated heat units for timing defoliation for optimum yields have varied tremendously from year to year. It has been suggested that the actual temperatures of the developing bolls in the canopy may not be closely represented by ambient temperatures measured at a meteorological site. The objective of this field study was to compare the current method of timing defoliation using ambient air temperature with actual boll temperatures. Internal boll temperatures were about 5°C warmer than ambient temperatures at midday and similar at night. There were no differences in boll temperature at different depths, 0.5 and 1.0 cm, in the boll, and there were no cultivar differences. A predictive equation (y=0.5298x+19.387) was formulated to determine internal boll temperatures using ambient temperature. Comparing these two methods of calculating heat units showed that using boll temperatures, 850 heat units were accumulated about 8 days earlier than when using ambient temperatures. Furthermore, 60% of the time the use of ambient temperature did not allow sufficient time to accumulate 850 heat units before the end of September. Studies in 2006 compared the two predictive methods to schedule defoliation and determine the effect on yield and quality.