A total of 154 strains from 22 species of Bifidobacterium, Peptostreptococcus, Lactobacillus, Ruminococcus, Coprococcus, Eubacterium, and Fusobacterium, which are present in high concentrations in the human colon, were surveyed for their ability to ferment 21 different complex carbohydrates. Plant polysaccharides, including amylose, amylopectin, pectin, polygalacturonate, xylan, laminarin, guar gum, locust bean gum, gum ghatti, gum arabic, and gum tragacanth, were fermented by some strains from Bifidobacterium, Peptostreptococcus, Ruminococcus, and Eubacterium species. Porcine gastric mucin, which was fermented by some strains of Ruminococcus torques and Bifidobacterium bifidum, was the only mucin utilized by any of the strains tested.