An investigation was carried out to determine whether variations of dietary carbohydrates could modify the colonic flora in rats. Sprague-Dawley rats were fed with two equicaloric diets based on the AIN-76 diet ( American Institute of Nutrition 1977) but differing from that diet in content of carbohydrates, i.e. high sucrose (64%) or high corn starch (64%). Feeding was continued for 9 months ad libitum and no variation in weight gain was recorded among the different diets. A prevalence of aerobes, and a significant reduction in the ratio anaerobes/aerobes in the faeces of rats on the high starch diet compared with the high sucrose diet, was observed. The anaerobe genera identified included Actinomyces, Bacteroides, Bifidobacterium, Clostridium, Eubacterium, Lactobacillus and Propionibacterium. Bacteroides was the most prevalent genus in both dietary groups (51·2 and 29·5% in the faeces of rats fed the sucrose and starch diets, respectively). In contrast, clostridia were prevalent in the starch-fed group (23·8%) and less so in the sucrose diet (11·5%), as propionibacteria were prevalent in faeces of rats fed the starch diet (15·5%), and low in the sucrose diet (3·9%). The remaining genera were scarce in faeces from rats on either diet. Total short-chain fatty acids (SCFA) were significantly higher in the faeces of animals fed the starch diet compared with those fed the sucrose diet. The relative concentrations of acetic, propionic and butyric acids were not significantly different between the two dietary groups. In conclusion, high starch diet can markedly modify the composition of faecal flora and alter considerably the faecal concentration of SCFAs, compounds which might have a health-promoting effect.