Diet composition has been suggested as a factor that influences the variability of responses when Saccharomyces cerevisiae (SC) is fed to ruminant animals. Diets based on lucerne hay (462 g/kg DM) and maize silage (488 g/kg DM) were fed to determine the effects of 0 or 5 g SC 47 (8´109 cfu/g) on ruminal digestion, fermentation and protozoa population. Ruminal pH, acetate, propionate, degradation rate and effective degradability were significantly affected by the forage sources. The addition of SC caused an increase in degradability of forage neutral detergent fibre (NDF), and tended to enhance degradability of total diet organic matter (OM), and the concentration of propionate and result in a decrease in protozoa numbers at 3 h post feeding. Ruminal crude protein degradation, ammonia-N concentration, acetate : propionate ratio and pH were not elicited by the addition of SC. Although the ruminal environment was significantly affected by the forage sources, no interaction between SC and forage sources occurred for ruminal digestion parameters: pH, ammonia-N and protozoa populations. However, compared with the maize silage, the SC increased the initial degradability (3 h after feeding) of forage NDF (4.6% vs. 1.7%), total diet OM (3.1% vs. 1.0%) and crude protein (CP) (5.5% vs. 0.1%) to higher proportions for lucerne hay. Moreover, acetate concentration was increased on the diet based on maize silage and decreased on the diet based on lucerne hay with supplementation of SC. Although ruminal environments were considerably altered by the forage sources, the SC exhibited a transitory effect (3 h post feeding) without overall improvement on ruminal digestion and fermentation; nevertheless, this observation was more pronounced for lucerne-based diet.