Pittman, K. A. (Agricultural Research Service, Beltsville, Md.), and M. P. Bryant. Peptides and other nitrogen sources for growth of Bacteroides ruminicola. J. Bacteriol. 88:401–410. 1964.—Representative strains of Bacteroides ruminicola were found to utilize peptide nitrogen or ammonia nitrogen, but not to utilize significant amounts of free amino acid nitrogen or the nitrogen from a variety of other low molecular weight compounds for growth. All strains grew well in a defined medium containing glucose, minerals, B-vitamins, heme, volatile fatty acids, methionine, and cysteine, with ammonia as the main nitrogen source. Methionine and cysteine were required by some strains. The only compounds found to replace ammonia as the main nitrogen source were a few proteins; tryptic digests of protein; peptide-rich fractions of Sephadex G-25 fractionated tryptic digests of casein; and the octapeptides, oxytocin and vasopressin. Most of the nitrogen present in these compounds was utilized. However, the organism did not utilize nitrogen from any of 12 dipeptides, triglycylglycine, glutathione, or mixtures of free amino acids. Possible reasons for the inability of B. ruminicola to utilize low molecular weight nitrogen compounds are discussed.