Growth of potato plants (cv. Desiree) was analyzed over a seven-year period in commercial fields at two locations, during Israel's two growing seasons. The effect of temperature on developmental rate was studied to determine the appropriate heat-unit model for predicting dates of tuber appearance, and termination of the yield-bulking period. The best prediction of tuber appearance date was based on the accumulation of heat units calculated using a linear relationship between developmental rate and daily mean temperature with a combination of 2°C-25°C-35°C for threshold, optimum and maximum temperatures, respectively. Predictions of bulking cessation based on several reported models were not any more accurate than predictions based on the accumulation of chronological days. High temperatures were found to negatively affect the developmental rate at both developmental stages. However our analyses indicate that the maximum temperature for tuber appearance is higher than that reported previously.