The concept is widely held that a “normal microflora” lives on the skin and mucous membranes of the upper respiratory, genitourinary, and intestinal tracts of man and other animals. The term “normal microflora” may mislead, however, as an encompassing description of the microbial populations to be found on body surfaces and in the contents of certain areas of the intestines. Microorganisms of numerous species have been seen in close association with epithelial surfaces in all parts of the gastrointestinal tracts of putatively normal laboratory rodents and chickens. The microorganisms found in the systems can be recognized to be either autochthonous species, or allochthonous species only passing through the region or temporarily colonizing a habitat vacated by its autochthonous inhabitants for one reason or another. Microorganisms of many genera and species have been found associating closely with epithelial surfaces of mucous membranes in various areas of the bodies of animals of numerous species including man.