The objective of this study was to determine whether Aspergillus oryzae fermentation extract stimulated or inhibited ruminal fermentation when fed at higher than recommended doses (3 g/d). Four dietary treatments of A. oryzae fermentation extract were fed daily to six cows fitted with ruminal cannulas. For each of four periods, bromegrass hay (6% CP) with and without extract was fed for 28 d. Dacron bags containing bromegrass cell walls were ruminally incubated to determine ruminal fiber degradation. The A. oryzae fermentation extract did not affect degradation of cell walls, cellulose, or hemicellulose. Total ruminal anaerobic or cellulolytic bacteria were not different among treatments; neither were the proportions of cellulolytic species, Butyrivibrio sp., Ruminococcus albus, or Ruminococcus flavefaciens. Ruminal ammonia was not different; however, total VFA were higher, and pH tended to be lower, when 27 g/d of A. oryzae fermentation extract was fed. The proportion of VFA was not different among treatments. The A. oryzae fermentation extract fed at nine times the recommended dosage did not produce any stimulatory effects, except for total VFA, and was not inhibitory or toxic to ruminal metabolism and forage fiber degradation.