Anaerobic rumen bacteria were screened for growth inhibition by methylamine. Under nitrogen-limited growth conditions only Ruminobacter amylophilus, Prevotella ruminicola and Megasphaera elsdenii were significantly inhibited. Ruminobacter amylophilus was inhibited more than any other species. Inhibition of R. amylophilus growth occurred at all increments in methylamine for all NH,CI concentrations tested (0.2-1.8 mm). Dimethylamine was less inhibitory, while trimethyl-, ethyl- and diethylamine were not inhibitory. When methylamine was added to in vitro mixed culture, ruminal fermentations containing alfalfa, neutral detergent fiber disappearance and ammonia production were not affected. This suggests that although methylamine may inhibit specific rumen bacteria, it does not alter overall mixed ruminal microorganism activities with alfalfa as a substrate.