Quantitative studies of the fecal flora of populations at different risk of colon cancer indicate that the relative proportions of some particular species of bacteria rather than of different genera of bacteria may be correlated with colon cancer incidence. Since the metabolic activity of different species in each genus varies widely, a shift in proportions of species could cause a major change in total metabolic activity in the individual. In samples taken from various areas of the intestinal tract and in scrapings from the intestinal wall, the composition of the flora remained relatively constant from the ascending colon to the rectum. Therefore, the bacteria in feces do reflect the flora of the large colon. The composition of the flora was not significantly affected by drastic changes in diet, but statistically significant shifts in the proportions of some species were noted in individuals under conditions of anger or fear stress. Although diet may not change the flora the individual maintains, the bacteria present may convert the different substrates provided by a high-fat diet as opposed to a high-fiber diet into metabolites that are potentially carcinogenic. The conversion of dietary components to carcinogenic compounds, identification of the bacteria capable of effecting such conversions, and the conditions favoring the proliferation of such bacteria will be investigated in greater detail.