This study was conducted to determine if adding relatively large quantities of a rapid growing, starch fermenting rumen bacterium, Prevotella bryanti, strain 25A, to the rumen of dairy cows would modulate rumen fermentation to reduce the risk of sub-acute ruminal acidosis (SARA). Twelve rumen fistulated dairy cows were fed an energy dense barley diet during a 3 week pre-partum period and then for 7 weeks post partum. Cells of P. bryantii that had been grown in a 100 liter fermenter were stored as frozen cells and then added daily to the rumen of six of the cows. Cows receiving the inoculum had lower post-feeding ruminal lactic acid, higher ruminal acetate and butyrate, and higher production of milk fat. However, since neither experimental nor control cows developed signs of SARA under the feeding conditions of this experiment, conclusions about protection against SARA by P. bryantii can not be reached.