Motion picture studies by Reichenbach of swarms of the myxobacteria Chondromyces apiculatus show that a swarm can sense the presence of another at a distance, and that the two swarms will approach each other ( Reichenbach, 1965). Our hypothesis for explanation of this behavior assumes the secretion of a diffusible chemotactic attractant by the bacteria, but not in the expected sense that the attractant produced by one swarm directly attracts the other. According to the hypothesis, an isolated swarm retains its integrity through the dynamic balance of random movement outward and chemotactic movement inward. The presence of another swarm nearby upsets this balance by altering the chemical gradient, allowing cell migration in directions in which the gradient is diminished. Thus, the primary mechanism for swarm approach may be random movement which predominates when chemotactic gradients are altered by the presence of a nearby swarm. Using a mathematical model, the swarm behavior predicted by this hypothesis is determined and quantified, including speed of migration, bounds for distance of interaction, swarm fringe activity, and nature of the proposed attractant.