In recent years there have been some startling advances made in the taxonomy of bifidobacteria. The bifidobacteria are classified as a distinct genus Bifidobacterium, and a number of species and biovars are now recognized. As better techniques for the study of bacteria are devised, more definitive information on importance of bifidobacteria as part of the intestinal flora of humans and animals has been obtained. Some ecological relationships of this organism are reviewed in this paper.