An ant mill is an observed phenomenon in which a group of army ants, separated from the main foraging party, lose the pheromone track and begin to follow one another, forming a continuously rotating circle. This circle is commonly known as a "death spiral" because the ants might eventually die of exhaustion. It has been reproduced in laboratories and in ant colony simulations.^[1]

The phenomenon is a side effect of the self-organizing structure of ant colonies. Each ant follows the ant in front of it, which works until a slight deviation begins to occur, typically by an environmental trigger, and an ant mill forms.^[2] An ant mill was first described in 1921 by William Beebe, who observed a mill 1200 ft (~370 m) in circumference.^[3] It took each ant two and a half hours to make one revolution.^[3] Similar phenomena have been noted in processionary caterpillars and fish.^[4]