Models were developed to describe the effects of heating rate during heat treatments on the mortality of 5th-instar codling moth, Cydia pomonella (L.). An old model, developed from previous studies over a limited range of heat treatments, was 1st formulated. Subsequent heat treatments, using a computerized water bath system and linear heating rates of 4, 6, 8, 10, and 12 degrees C/h at 42, 44, and 46 degrees C, were used to test the old model. The mortality data from the water bath study were used to develop a new model. Although the old model provided a good estimate of the effects of heating rate on 5th-instar mortality, it overestimated mortality at midrange heating rates. Also, the old model was awkward to use because it required a correction for each treatment temperature. The new model incorporated treatment temperature into the equation, and was more accurate and easier to use. It was determined that the slower the rate of heating, the longer the exposure to the final treatment temperature was needed to achieve 95% mortality.