Ammonia saturation constants were determined for representative pure cultures of predominant, anaerobic, fermentative rumen bacteria. Based on growth experiments with ammonia limited continuous cultures, average estimates for ammonia saturation constants of Bacteroides amylophillus and Bacteroides ruminicola were 10.5 and 23.5 microM ammonia-nitrogen, respectively. With ammonia-limited linear-growth cultures, the estimates for the ammonia saturation constants of B. amylophilus, B. ruminicola, and Selenomonas ruminantium were, respectively, 6, 33.5, and 18 microM ammonia nitrogen. By a third method, which involved estimation of ammonia concentration in the medium when the growth rate of ammonia-limited batch cultures reached half maximal, the ammonia saturation constant was determined for the species mentioned as well as Megaspaera elsdenii and Ruminococcus flavefaciens. Except for M. elsdenii, saturation constants of the other bacteria were less than 50 microM ammonia nitrogen. An organism with a saturation constant for ammonia of 50 microM growing in a medium containing 1 mM ammonia should achieve 95% of its maximum specific growth rate. Many of the predominant species of rumen bacteria are efficient scavengers of ammonia.