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We studied the migration through semisolid agar of chemotactic and nonchemotactic cells of Escherichia coli. While swarms of nonchemotactic cells were generally smaller than those of chemotactic cells, they varied markedly in size and in structure. Cells that failed to tumble or that tumbled incessantly formed the smallest swarms. Cells that tumbled at intermediate frequencies formed much larger swarms, even when deleted for many of the genes known to be required for chemotaxis. Surprisingly, the higher the tumble frequency, the larger the swarms. Microscopic examination revealed that tumbles enable cells to back away from obstructions in the agar. Thus, not all cells that swarm effectively need be chemotactic.