Polysaccharides with structures resembling components of dietary fiber were fermented by a number of species of anaerobic bacteria from the human colon. Some strains also fermented glycoprotein mucins. The strains that fermented the widest range of polysaccharide substrates were in the two genera Bacteroides and Bifidobacterium. Polysaccharide degrading enzymes from several Bacteroides species have been studied, and in most cases the enzyme activities were cell bound rather than extracellular. In all cases, the polysaccharide degrading enzymes were inducible rather than constitutive. Thus the metabolic activity of the flora could be altered considerably by the amount and type of fiber in the diet, even though the composition of the flora itself remained unchanged. The products of enzyme action included monosaccharides and oligosaccharides of varying chain lengths.