A study has been made of the distribution of bacterial flora in the gastro-enteric tract of rats fed on glycidic, protein and lipidic diets and compared with that found in rats given a standard diet. The flora of the gastro-enteric tract was noted in a fraction adhering to the epithelial cells of the mucous membrane and in a fraction present in the contents of the gastro-enteric tract. Changes resulting from the differences in diet are made particularly at the expense of the adhering flora. In groups fed on a glycidic diet there is a clear prevalence of enterococci, enterobacteria, lactobacilli, peptostreptococci and yeasts. In rats fed on a protein diet the quantity and variability fo the flora are less and clostridia predominate. Where rats are given a lipidic diet there is a prevalence of enterobacteria, lactobacilli and peptostreptococci. An extensive and diversified colonization is noted as a further consequence of differences in diet. The results obtained are discussed in relation to certain pathological intestinal conditions and to results reported elsewhere in the literature.