The aim of the study was to determine the effect of live yeast culture (LYC) and dehydrated yeast culture (YC) containing Saccharomyces cerevisiae on fermentation parameters and degradability in the rumen of sheep fed a high- (H-NDF; 56% DM) or low- (L-NDF; 35% DM) NDF diet. Three Polish Merino sheep fitted with ruminal cannulas were used. The effective degradability of maize grain and wheat straw was measured by an in situ procedure. Ruminal pH, ammonia, total volatile fatty acid concentrations, and molar percentages of acetate, propionate and butyrate were not modified by the addition of LYC or YC, irrespective of the applied diet. Compared with control (CON) and LYC supplementation of the L-NDF diet significantly (P<0.05) increased the caproate concentration, but in the H-NDF diet, it increased the lactate concentration (P<0.05). In situ rumen NDF degradability of maize grain and wheat straw was not affected by the treatments. When the sheep were fed the H-NDF diet, however, degradability of dry matter maize grain was significantly higher (P<0.05) than in the CON group. The results of this study did not confirm the hypothesis that NDF in the diet is the main reason for the considerable inconsistencies in the results of experiments with Saccharomyces cerevisiae products.