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Environmental heat and work-rate are risk factors for Heat Related Illness (HRI). Work-rate by task and core temperature have not been quantified in California farmworkers.Farmworkers were monitored for one work-shift each in the summers of 2014–2015. Individuals’ core temperature was assessed throughout the shift using an ingestible sensor, a 3 min moving average computed and maximum temperature identified. Accelerometers were worn, and NHANES criteria used to classify counts per minute (cpm) into sedentary, low, moderate and vigorous activity. Daily work-rate was categorised by the number of minutes spent in moderate and/or vigorous activity (<30, 30 to 90,>90). Questionnaires administered in Spanish collected occupational tasks conducted and self-rated environmental heat exposure.499 Latina/o farmworkers performed only one task on their shift. The mean activity in cpm was highest for tree/vine harvesters 445 (SD 225) and lowest for produce sorters 193 (SD 167). 22 workers recorded a maximal core temperature >38.5°C, a criteria for heat stress in acclimatised workers. In a multivariable logistic regression high body temperature was associated with both the number of minutes working at a moderate/vigorous rate and self-rated environmental heat; OR and (95% CI) for ≥90 v<90 min high activity 3.6 (1.5–8.5). Irrigators were the only classification with statistically significant association with elevated core temperature; OR and (95% CI) 3.7 (1.4–9.6).Farmworkers, who exceed 90 min a day in moderate/vigorous activity and/or irrigators, are at higher risk of HRI. These workers may need closer monitoring for their safety.