Yeast cultures, especially Saccharomyces cerevisiae, are beneficial in the rumen, and have been progressively introduced into the feed industry. They can affect microbial activities, thus improving fiber digestion, modifying VFA production and increasing animal performance. Most of studies have focused on yeast effects when good quality and high concentrate diets are used, but much less attention has been paid to the possible effects with fibrous feeds. In this study, the effects of a preparation of S. cerevisiae (strain SC-47) on in vitro ruminal fermentation of 4 fibrous feeds differing in quality and chemical composition were analyzed. Two low-quality forages (barley straw and Pennisetum purpureum clon Cuba CT-115) and 2 medium-quality forages (alfalfa hay and grass hay) were used. Samples of the forages (500 mg) were accurately weighed into 120-mL serum bottles, and incubated with buffered rumen fluid (50 mL) at 39°C for 24 h. Based on previous experiments with the same product, 4 doses of the S. cerevisiae preparation were tested as feed additive in batch cultures at 0, 25, 50 and 100 µL/bottle. VFA and gas production, pH, ammonia-N concentrations and true in vitro digestibility (TIVD) were measured. There were no additive x substrate interactions (P > 0.05) for any variable. TIVD, ammonia-N concentration and acetate:propionate ratio were not affected (P > 0.05) by SC-47 addition to the batch cultures. Gas production and total VFA production were linearly increased (P < 0.001) by increasing amounts of SC-47, whereas pH was linearly decreased (P = 0.010). Also, the production of all individual VFA linearly increased with the addition of the different doses of SC-47 (acetate, P = 0.001; propionate, P < 0.001; butyrate, P < 0.001; and the sum of isobutyrate, isovalerate and valerate, P < 0.001). The results show that S. cerevisiae stimulate ruminal fermentation in vitro of these fibrous forages of different quality, and so, studies analyzing their possible influence on fermentation of low-quality substrates are recommended.