We describe a new behavioral response in Escherichia coli and Salmonella typhimurium in which the bacteria differentiate into filamentous, multinucleate, hyperflagellate cells that navigate the surface of solid media by means of coordinated swarming motility. The cue for differentiation into swarmer cells is provided by the concentration and composition of the agar. Examination of the behavior of various mutants shows that the flagellar apparatus used for swimming motility and the chemotaxis system are indispensable for swarming motility.