Environmental heat stress, expressed as the Wet Bulb Globe Temperature (WBGT), was measured outdoors using three different instruments: a) the conventional shaded dry-bulb, 15.2-cm black globe and naturally convected wet bulb thermometers, b) a minaturized thermometer kit, and c) a commercial WBGT instrument using a thermistor sensors. The WBGT values were compared with the Wet Globe Temperature (WGT) measured with a Botsball. Measurements were made visually on the instruments at regular intervals and an automated data collection system also was used to obtain data from thermocouples attached to the instruments. Statisticallly significant differences in WBGT readings were found among the instruments; however, the difference for a given environment usually was less than 0.5 degrees C. Readings taken by visual observations resulted in WBGT values which differed by less than 0.3 degrees C from those calculated from the automated data collection system. By using an equation derived for the Botsball, WBGT = 1.044 WGT - 0.187 (in degrees C), it is possible to convert the Botsball thermometer dial to indicate the conventional WBGT for outdoor environments, thus making it a simple instrument for assessing environmental heat stress at the work site.