Glutamate and aspartate are key amino acids for catabolism by Fusobacterium nucleatum subspecies and Porphyromonas gingivalis respectively. However, peptides such as yeast extract are their preferred sources of energy. To determine more precisely the possible nature of these peptides, we examined the utilization of these amino acids and their corresponding peptides by cell suspension experiments with a fluorescamine labeling technique. High molecular weight (M.W.) polyglutamate (> 40,000) was poorly utilized by all taxa, whereas 95% of its low-M.W. peptide (2,000-5,000) was used by F. nucleatum subspecies nucleatum, but the remaining two subspecies utilized < 30%. P. gingivalis used ca. 50% polyglutamate but > 90% polyaspartate within the same period. For F. nucleatum subspecies nucleatum as the test organism, T0.5 (the time taken to use 50% of the test substrate) was 1.7 h longer for glutamate than for the homopolymer. Furthermore, in the presence of both substrates, polyglutamate suppressed the uptake of glutamate until about 50% (ca. 1.5 mmol/L) of the peptide was used, after which the incorporation of the free amino acid started. A similar pattern of utilization was observed in P. gingivalis with its preferred peptide polyaspartate, for which the T0.5 was three times shorter than its monomer, aspartate. Both species had the capacity to utilize the heteropolymer, poly aspartate/glutamate, but at a significantly slower rate than the corresponding homopolymer.