We demonstrate that bumble bees carrying heavier pollen loads have hotter body temperatures. We sampled wild, freely foraging Bombus impatiens workers in Raleigh, NC, USA, and measured their thoracic temperature, body weight, and pollen load weight. After accounting for bee weight and local operative temperature, bee thoracic temperature increased 0.07 C per mg pollen carried. For a full pollen load, the estimated total effect was about 2 C. On hot days, this effect could push a foraging bee from a safe body temperature to a highly stressful one, within the range of the critical thermal limits we measured for Bombus impatiens. Future research is needed to document how bumble bees adjust their foraging strategies to avoid acute heat stress, and whether such changes have costs at the colony level.