The extracellular polysaccharides (EPSs) produced by 37 isolates presently classified as Butyrivibrio species (or more specifically as Butyrivibrio fibrisolvens) were purified from glucose-grown cultures. The neutral sugar compositions of these EPSs were determined by both thin-layer and gas-liquid chromatographic techniques. Results showed that while the neutral sugar composition of the EPS was constant for a given strain, it varied considerably between strains. In addition, several acidic components in the EPS, of both known and unknown structure, were detected artifactually as acetylated lactones, the acetylated alditols derived from these lactone(s), or both. Two novel components, L-altrose and the acidic sugar 4-O-[1-carboxyethyl]-D-galactose, were common constituents of the EPS from some strains of B. fibrisolvens. These and other EPS compositional features were used to sort isolates of B. fibrisolvens into groups which may have taxonomic significance. A scheme for sorting isolates into these groups, and the relative relationships between groups, is proposed.