AbstractFor 30 days, male weanling rats were fed a semipurified, fiber‐free diet or a diet that contained 5, 15, or 30% (wt/wt) wheat bran. The activities of four cecal microbial enzymes were determined. Wheat bran significantly increased the wet weight content of the cecum and total bacterial count per cecum at the intermediate‐and high‐treatment levels, but it had no effect on bacterial concentration per gram wet weight of cecal contents. Totalβ‐glucosidase andβ‐glucuronidase activities per cecum were generally increased. Wheat bran decreased total nitrate reductase activity, but there was no change in total nitroreductase activity. Wheat bran significantly decreased enzyme activities for nitro‐and nitrate reduction per gram of cecal contents but increasedβ‐glucosidase activity. The activities of the enzymes per 1011bacteria followed a similar pattern to that noted per gram of cecal contents. Such fiber‐dependent changes in enzyme activity may alter the steady‐state concentration of toxic and genotoxic chemicals in the lumen of the hindgut.