A simple model is described wherein ant foragers choose a foraging sector as a function of the pheromone concentration associated with each sector. The choice is autocatalytic, as foragers that find food in a sector add to its pheromone. As a sector's food runs out, the foragers spontaneously switch to the adjacent sector. With increasing food abundance, the model passes from random foraging to the formation of a trail that rotates about the nest. The greater the abundance the more slowly the trail rotates until it finally becomes fixed on one sector. These results agree with experimental observations made on the harvester antMessor pergandei byBernstein (1975) andRissing andWheeler (1976), and reconcile an apparent contradiction between them.