The effect of dietary fiber on colonic carcinogenesis was studied endoscopically in rats administered 1,2-dimethylhydrazine. Colonic tumors appeared about six weeks later in rats fed 15% cellulose diet or 40% wheat bran diet than in those fed basal diet. Furthermore, the incidence of colonic tumor was significantly lower from 23th week to 26th week in the cellulose diet group (p less than 0.01) and at 26th week in the wheat bran diet group (p less than 0.05) than in the basal diet group. However, the incidence of colonic tumors and the mean number of tumors per rat at sacrifice (30th week) were not significantly different between these two high-fiber diet groups and the basal diet group. These two diets significantly increased feces 3 to 4 times in weight and 5 times in volume over the base diet. However, 15% pectin diet could neither inhibit colonic carcinogenesis nor increase the fecal weight. These results suggested that cellulose and some dietary fibers of wheat bran were effective to retard colonic carcinogenesis, and that the increased fecal weight and volume by fibers may be involved in the inhibitory effect.