Since the discovery of Shigella as the aetiologic agent of acute dysentery almost 100 years ago, this organism has been described as a non-motile and nonflagellated organism that invades the human colonic mucosa. In this study, the production of flagella by prototypic strains of all four Shigella species and, moreover, by fresh clinical isolates was demonstrated by electron microscopy. The flagellum of Shigella (flash) is approximately 10 microns long and 12-14 nm in diameter and is typically seen emanating from one pole of the bacterium. Flash is composed of a putative structural polypeptide subunit of 33-38 kDa that shares immunological similarities with Escherichia coli, Salmonella spp., and Proteus mirabilis flagellins, and with the recently described recombinant Shigella flagellins (FliCSS and FliCSF) expressed in E. coli K-12. A fliCSS-specific oligo probe hybridized with all four Shigella species, while a fliCSF probe hybridized with all Shigella flexneri and Shigella dysenteriae strains, but not with all Shigella sonnei or Shigella boydii strains, indicating genetic divergence among their flagellin genes. Shigella exhibits motility in low-concentration motility agar under physiological growth conditions. The expression of flash and motility appears to be strictly regulated by unidentified genetic and environmental factors. These heretofore undescribed features may allow the bacteria to circumvent the natural intestinal mucosal defences leading to bacterial colonization and disease. The motility of shigellae may represent an evolutionary adaptation important for bacterial survival.