Ingestion of high levels of rapidly fermented carbohydrates after parturition often leads to the production of excessive quantities of organic acids that may exceed the buffering capacity of the rumen and cause pH to drop. Ruminal acidosis results in animal discomfort, anorexia, depression, decreased digestibility, and decreased milk production. In the present study, we examined the effects of daily addition of cells of a newly isolated strain of Prevotella bryantii (25A) to the rumen of 12 ruminally cannulated cows in early lactation. This strain was selected based on earlier in vitro studies that indicated its ability to grow rapidly, compete for starch, and produce organic acids other than lactate. After calving, all cows received increasing amounts of an energy-dense diet containing barley grain, corn silage, and grass silage in a 40:60 forage-to-concentrate ratio. Animals were blocked according to milk production from their previous lactation. Treatments (control and P. bryantii) were distributed among cows within the same block. Cows were fed once a day. Six cows were given a daily dose of P. bryantii (2 x 10(11) cells/dose), administered directly with a syringe through the rumen cannula, from 3 wk prepartum up to 7 wk postpartum. Rumen fluid was sampled before feeding and at 2 and 3 h postfeeding on wk 1, 2, 3, 4, 6, and 7 postpartum. Feed intake and milk yield were recorded daily and milk composition was recorded 2 d/wk, up to wk 7 of lactation. Feed intake was similar between control and treated cows. Prevotella bryantii did not change milk production, but milk fat tended to be greater in treated cows compared with control cows (3.9 vs. 3.5%). Rumen pH was similar between the 2 groups and differed across sampling times, being higher before feeding (6.3) as opposed to 2 h (5.9) and 3 h (5.7) postfeeding. Rumen lactate concentration was similar before feeding between control and treated cows; however, 2 to 3 h after feeding, lactate concentration was lower in cows receiving P. bryantii compared with control cows (0.7 vs. 1.4 mM). This difference was maintained throughout the experimental period. Concentration of NH(3)-N was greater in treated cows than in control cows (174 vs. 142 mg/L). Acetate (65.5 vs. 57.8 mM), butyrate (12.7 vs. 10.5 mM), and branched-chain C4 fatty acid (0.90 vs. 0.75 mM) concentrations were greater in postfeeding samples of treated cows compared with control cows. Supplementing early-lactating cows with P. bryantii 25A increased ruminal fermentation products and milk fat concentration. Because signs of subacute ruminal acidosis were not observed in either treated or control cows, no conclusions can be made about possible protection against acidosis by P. bryantii.