Swarming by Proteus mirabilis is characterized by cycles of rapid and coordinated population migration across surfaces following differentiation of vegetative cells into elongated hyperflagellated swarm cells. It has been shown that surface colony expansion by the swarm cell population is facilitated by a colony migration factor (Cmf), a capsular polysaccharide (CPS) that also contributes to the uropathogenicity of P. mirabilis (Gygi, D., Rahman, M. M., Lai, H.-C., Carlson, R., Guard-Petter, J., and Hughes, C. (1995) Mol. Microbiol. 17, 1167-1175). In this report, the Cmf-CPS was extracted with hot water, precipitated with ethanol, and further purified by gel permeation chromatography. Its structure was established by glycosyl composition and linkage analyses, and by one- and two-dimensional NMR spectroscopy. The Cmf-CPS is composed of the following tetrasaccharide repeating unit. [see text]