Newly weaned rats fed 12 weeks on a diet containing no dietary fiber or no fiber except for 10% cellulose, maintained the leaf-like intestinal villous morphology present at weaning, as observed by scanning electron microscopy. In rats on a normal laboratory diet the jejunal morphology showed progression from the leaf-like villous pattern at weaning to broad-leafed, long-ridged villi of adulthood. Pectin added to a no-fiber diet caused structural changes similar to but less well developed than those changes in the rats on a standard diet. Striking differences were noted not only in the appearance of the intestinal villi but in the number of villi per square centimeter between those animals on no fiber or no fiber except cellulose and those animals on pectin or standard diets. Cholestyramine, a strong pharmacological bile salt-binding agent, when added to a no-fiber diet, did not promote development of the usual villous pattern, and the structure remained the same as that in rats on no-fiber and cellulose diets. Cellulose (no bile salt-binding capability) and pectin (weak bile salt-binding capability) added to a no-fiber diet were associated with significant differences in the number of villi in both the jejunum and the ileum. The observed changes in morphology are unlikely to be due to differing bile salt-binding capabilities of different fiber substances.