Environmental Health, Japan-Objectives: Measuring core body temperature is crucial for preventing heat stress disorders in workers. We developed a method for measuring auditory canal temperatures based on a thermocouple inserted into a sponge-type earplug. We verified that the tip of this thermocouple is positioned safely, allowing the wearer to engage in normal physical tasks; this position averaged 6.6 mm from the tympanic membrane. Methods: To assess this technique, we had six healthy male students repeat three cycles of exercise and rest (20 min of exercise and 15 min of rest) in a temperaturecontrolled chamber with temperatures set at 25, 30, or 35°C, while monitoring the auditory canal, esophageal, rectal, and skin temperatures. Results: We observed differences of a mere 0.30-0.45°C between rectal temperatures and auditory canal temperatures measured with the thermocouple, the smallest such difference reported to date in studies involving auditory canal temperature measurement. Conclusions: We conclude that monitoring temperatures based on a technique involving an auditory canal plug can be used to estimate rectal temperatures accurately, and thereby to avoid conditions leading to heat stress disorders. Although controlling ambient temperatures is the ideal approach to eliminating heat stress disorders, this is often not possible. This makes it especially important to optimize work schedules by reducing hours worked, providing workers with frequent breaks, or having workers work in alternating shifts. Ideal work schedules should be based on evaluations of physiological response, including perspiration rates and heat balance, as defined in the ISO7933 Predicted Heat Strain model 1) . In practice, however, measuring the parameters required for this model poses major difficulties. The indices available for workplace evaluations include various proxies for core temperature (t cr ) measurements, including measurements of skin temperature (t sk ), heart rate (HR), and body weight. Monitoring t cr in vivo is essential to preventing heat disorders.According to ISO 9886 2) , the term "core" refers to all tissue located at depths sufficient to remain unaffected by the temperature gradient existing on surface tissue. Esophageal temperature (t es ), rectal temperature (t re ), intraabdominal temperature (t ab ), oral temperature (t or ), tympanic temperature (t ty ), auditory canal temperature (t ac ) and urine temperature (t ur ) have been proposed as core temperature indices. The transducer of t es is placed in the lower part of the oesophagus, which is in contact over a length of 50 to 70 mm with the front of the left auricle and with the rear surface of the descending aorta. Consequently, t es reflects the temperature of the arterial blood with a very short reaction time. t re is independent of ambient conditions because the rectum is surrounded