This report describes a new behavioural response of Bacillus cereus that consists of a surface-induced differentiation of elongated and hyperflagellated swarm cells exhibiting the ability to move collectively across the surface of the medium. The discovery of swarming motility in B. cereus paralleled the isolation of a spontaneous non-swarming mutant that was found to carry a deletion of fliY, the homologue of which, in Bacillus subtilis, encodes an essential component of the flagellar motor-switch complex. However, in contrast to B. subtilis, the fliY mutant of B. cereus was flagellated and motile, thus suggesting a different role for FliY in this organism. The B. cereus mutant was completely deficient in chemotaxis and in the secretion of the L2 component of the tripartite pore-forming necrotizing toxin, haemolysin BL, which was produced exclusively by the wild-type strain during swarm-cell differentiation. All the defects in the fliY mutant of B. cereus could be complemented by a plasmid harbouring the B. cereus fliY gene. These results demonstrate that the activity of fliY is required for swarming and chemotaxis in B. cereus, and suggest that swarm-cell differentiation is coupled with virulence in this organism.