Background: Heat health effects are an important public health problem in outdoor workers, including agricultural workers. Outdoor agricultural workers who perform heavy physical labor in hot conditions are at increased risk for developing occupational heat-related illness. Heat stress, under certain environmental conditions, has been reported to reduce worker productivity. Climate models project future increases in the frequency, severity, and duration of heat waves. Objectives: This study aimed to characterize heat stress and physiological effects of heat stress (heat strain) in outdoor tree fruit workers performing harvest activities in Yakima Valley, Washington, and to assess the relationship between heat exposure and productivity in these workers. Methods: During the summer of 2015, 46 pear and apple harvesters from six orchards participated in a cross-sectional study in Yakima Valley, Washington for one work shift each during warmer periods in August (n=34 pear harvesters) and cooler periods in September (n=12 apple harvesters). All participants were paid by the amount of fruit harvested (piece-rate). Heat stress and strain were characterized using American Conference of Governmental Hygienist (ACGIH) guidelines, which recommend thermal Action Limits and Threshold Limit Values based on several factors, including environmental conditions, metabolic rate of task, and clothing ensembles. Heat exposure was measured near individual workers using hand-held Wet Bulb Globe Temperature (WBGT) monitors, metabolic rate was estimated using field observations and personal hip-mounted accelerometers, and research staff observed workers’ clothing. Heart rate and core body temperature were monitored over the course of the work shift using heart rate monitors and wireless ingestible core body temperature sensors. A computer-assisted self interview survey instrument captured other relevant demographic, individual, and work factors. The total weight of fruit bins collected per time worked was used to assess productivity. Effect estimates of the association between maximum work shift WBGT and productivity were estimated using linear mixed effects models with a random intercept for orchards, using Kenward-Roger methods for small sample sizes, adjusted for relevant confounders. Results: Surveys of workers indicated that 24 (52%) had experienced symptoms of heat strain and heat-related illness, and only 13 (28%) received training on working in the heat. Of the 34 participants who worked in pear harvest in August, 25 (74%) exceeded the ACGIH Action Limit (WBGTeffective 250 C), and 21 (62%) exceeded the Threshold Limit Value (WBGTeffective 280 C) for the moderate work task (300 Watts) of harvesting. Using personal accelerometer data to estimate metabolic rate (n=39), 12 (31%) participants exceeded the Action Limit and four (10%) exceeded the Threshold Limit Value. Of the 12 participants exceeding the Action Limit, based on accelerometer data, nine (75%) exceeded the maximum heart rate (180-age beats per minute), and five (42%) exceeded the maximum internal core body temperature of 38.5°C recommended by ACGIH. There was a trend of a decrease in productivity with increasing maximum daily WBGT, but this association was not statistically significant. Conclusions: Current summer tree-fruit harvesters in Yakima Valley, Washington are laboring in thermal environments hazardous to health. Payment schemes may provide incentives for workers to not slow down, and increase the risk of HRI. Acclimatization practices, HRI training, and orchard management practices could be improved to increase biological adaptation to heat stress and prevent HRI. The relationship between heat exposure and productivity in tree fruit harvesters is complex and likely affected by monetary factors and orchard and harvest characteristics. The effects of heat stress on heat strain and productivity in outdoor workers should be considered in future planning, given the projected increase in frequency, severity, and duration of heat waves.