Specimens of intestinal mucosa of rats were examined closely to determine the distribution of the mucosa‐associated microbial flora. Six distinct zones were found along the length of the intestinal tract. The first zone, in the small intestine 50 cm or more from the ileocaecal junction, had no associated flora; however, each of the five other zones had different populations of microorganisms associated with the mucosa. Some of these organisms have not been reported previously. Treatment of rats with oral doses of the purgative magnesium sulphate resulted in dramatic changes in the distribution of these mucosa‐associated microorganisms. The normal flora of ileal and caecal crypts disappeared, while the organisms in colonic crypts were unchanged. Large numbers of mucosa‐associated bacteria appeared in the stools of treated animals.