Factors leading to swarming of Vibrio alginolyticus cells on solid media were studied. Polar flagellated rods from liquid medium develop into small colonies on solid medium. Byproducts, accumulating in the colony area, induce at certain critical concentrations, the formation of peritrichous flagella and development of long heavily flagellated filaments which swarm away form the high by-product concentrations. Several types of nonswarming mutants were isolated, among them, mutants which lack the capacity to form swarming-inducing pyproducts, but can be induced to swarm by byproducts of other mutants incapable of swarming. Different compounds could replace the natural metabolic byproducts; at very low concentration these compounds induce peritrichous flagella and swarming in some of the nonswarming mutants mentioned above. The natural metabolic byproducts accumulating in yeast-extract-peptone medium are suggested to be volatile acids belonging to the valine and isoleucine pathway. Wild-type V. alginolyticus cells cannot swarm on certain substrates but its mutants, able to swarm on many substrates in minimal media, are easily selected.