The influence of a live culture of Saccharomyces cerevisiae on the number of ciliates, activity of carboxymethylcellulose- (CMC), xylan- and starch-degrading enzymes, as well as the concentration of VFA in the rumen of two cows was examined. The animals were fed either hay and concentrate alone or supplemented with live yeast given at the rate of 5 g/d. It was found that the response of rumen fauna to yeast supplementation was related to the animal. The total protozoa and Entodinia counts increased significantly only in one cow (P≤0.05). The number of protozoa from the genus Diplodinium remained unchanged (cow 1) or decreased significantly (cow 2). Epidinia decreased in the rumen of both animals, while Isotrichidae did not change significantly (P>0.05).The activity of CMC-ase and xylan-degrading enzymes varied between 7.2-9.4 and 108.3-130.0 μM released reducing sugars/g DM/h, respectively. The effect of yeast on the activity of fibrolytic enzymes was related to the host animal. CMC-ase decreased significantly only in cow 2, while xylanase, in cow 1. The activities of starch-degrading enzymes as well as pH of rumen fluid and volatile fatty acids were not affected by supplementation of cows with yeast.