Motility is one of the most extensively studied cellular events conducted by bacteria, including Escherichia coil. A motility agar plate assay showed that deletion of the rpoS gene enhanced the apparent motility of the E. coil BW25113 strain, which inherently had negligible motility compared to wild-type E. coli strains, such as MG1655, with no effect on cell growth. This enhancement of motility was accompanied by drastic up-regulation of genes involved in the formation and rotation of flagella. Furthermore, an individual cell motility assay showed that the population of Delta rpoS cells had bimodal motility character, and that a minority of this population exhibited a much higher motility rate. These results support a view that a minority population contributes to increasing in apparent motility of the whole population of Delta rpoS cells. (C) 2012, The Society for Biotechnology, Japan. All rights reserved.