ABSTRACT. The temporary nests (bivouacs), of Eciton burchelli (Westwood) thermoregulate with considerable precision. Temperatures at the core of the bivouac are maintained at 28.5±1d̀C even though ambient temperatures in their tropical rainforest home change during the day over a range of 7d̀C. Temperatures just under the mantle of the bivouac vary much more than at the core. Calculations suggest that the basal metabolic rate of the army ants in bivouac will generate all the heat that they need to keep warm when ambient temperatures drop. The army ants probably regulate heat loss by opening up ventilation channels within the nest.