Abstract Total aerobic and anaerobic microbial populations of the caecum were compared in male rats fed 0 or 7·5% sodium saccharin for 10 days. The weights of caecal tissue and caecal contents were higher in the saccharin-fed animals than in the controls. The presence of a high saccharin content, about 90 mg/ml, in the caecal contents did not alter the total numbers of anaerobic microbes, but resulted in the deletion of a specific anaerobic microbe which, when isolated from control samples, was shown to be sensitive to saccharin in vitro. Saccharin ingestion resulted in an increase in the numbers of aerobic microbes. Finally, it was demonstrated that saccharin (≥ 50 mg/ml of medium) could reduce the amount of ammonia produced from urea by Proteus vulgaris (ATCC 13315). These observations suggest a possible mechanism for the changes in urine previously noted in rats ingesting diets containing saccharin (Anderson, Fd Cosmet. Toxicol. 1979, 17, 195).