The influence of ammonia concentration on N-15-ammonia incorporation and de novo synthesis of amino acids by three predominant non-cellulolytic species of ruminal bacteria, Prevotella bryantii B(1)4, Selenomonas ruminantium HD4 and Streptococcus bovis ES1, was investigated. The medium contained pancreatic casein hydrolysate (comprising mainly peptides with some amino acids) at a concentration of 1 g/litre and additions of graded concentrations of (NH4Cl)-N-15. When the initial concentration of ammonia increased from 0.045 to 0.436 g N/L in the growth medium, the proportion of cellular nitrogen and amino acid nitrogen derived from ammonia by P. bryantii and S. ruminantium increased (ranging from 0.33 to 0.84 for cellular-nitrogen and from 0.23 to 0.67 for amino acid-nitrogen) (P<0.001). but S. bovis incorporated a fixed proportion of ammonia and peptides in all media except for the lowest ammonia containing medium (P>0.05). Glutamate and aspartate were the most highly labelled amino acids with N-15, whereas N-15 enrichment in proline was lower than that in other amino acids in all species, followed by phenylalanine in P. bryantii, lysine in S. ruminantium and phenylalanine, valine and lysine in S. bovis. indicating preferential incorporation of these amino acids from pancreatic casein hydrolysate. The results of the present study, thus, suggest that the concentration of ammonia has an important effect on de novo synthesis of bacterial cellular-nitrogen and amino acids in the non-cellulolytic ruminal bacteria and this effect depends on bacterial species.