Four ruminally fistulated Hereford steers (400 kg) were fed two isocaloric diets at 1.5 x maintenance once daily in a repeated measurement crossover experiment. Postprandial changes in hydrogen-oxidizing, carbon dioxide-reducing bacterial groups were monitored. The methanogenic bacterial populations were present at densities of 4 x 10(8) to 8 x 10(8)/g of ruminal contents on either the high- or low-forage diet. Numbers remained constant postprandially on the high-forage diet but showed a distinct rise and fall with the once-daily feeding of the low-forage diet. Presumed hydrogen- and carbon dioxide-utilizing, acid-producing (acidogenic) bacteria were present between 2 x 10(8) and 12 x 10(8)/g of ruminal contents, with the density of the low-forage population being twofold higher than that of the high-forage population. Acidogenic bacteria exhibited similar postprandial changes on both diets, with the predominant shift being associated with the feeding event. This is the first study which documents the postfeeding trends in ruminal methanogenic bacteria on specified, production-level diets. It is also the first study to suggest that other hydrogen-oxidizing, carbon dioxide-reducing bacteria which produce acid instead of methane are present at high population densities in the normally fed adult ruminant.