Three strains of B. thetaiotaomicron have been isolated from caeca of BALB/c-nu/+ mice of SPF. These Bacteroides are obligately anaerobic, Gram-negative, non-spore-forming and non-motil without flagella rods. A characteristic of staining is deep at ends and stainless at the medium of a rod. The isolation and identification of these strains has been reported in 1987. This paper introduced only the results of EM observations. Under the SEM, the unstained area of rods is always showing a concavity, which is just a nucleoid in sections under the TEM. Many lamellar corpuscles have been found in cell plasma. Some of them have been secreted out of the cells. The chemical properties and physiological functions of them are unknown.