In a previous study, we observed that administration of xylooligosaccharide selectively promoted the growth of intestinal bifidobacteria. In the present study we examined the relationship between intestinal bacteria and putrefaction products. Putrefaction products (p-cresol, indole, skatol) decreased according to the amount of xylooligosaccharide administered. An increase in the number of Bacteroides (spp.) caused an increase in the concentration of putrefaction products (ammonia, p-cresol, indole). On the other hand, an increase in the ratio of the number of Bifidobacterium (spp.) to the total number of intestinal bacteria caused a decrease in the concentration of putrefaction products (p-cresol, indole, skatol).