The effect of pectinolytic yeast culture Kluyveromyces marxianus CCY 51-1-1 on growth (total anaerobes, cellulolytic, amylolytic and pectinolytic bacteria) and enzyme activities (cellulase, amylase, pectinase and urease) of the rumen microflora were investigated. Significant increase of cellulase and α - amylase activities were found in experiment with 24 hours incubation in vitro in rumen liquid from calves or rams. Enhanced cellulase, α - amylase, pectinase activities and a decrease in urease activity were observed after three weeks of addition of yeast cultures to the diet of sheep. No significant effect on the rumen bacterial counts was observed in either type of the experiments. The concentration of yeast supplement was stable for eight hours when cultivated in vitro in rumen liquor.