This study aimed to evaluate, under in vitro conditions, the effect of the inclusion of yeasts of different species on some fermentation parameters of Tifton hay. The gas production technique was used and the strains evaluated belong to the species: Rhodotorula mucilaginosa, Issatchenkia orientalis, Candida tropicalis, Pichia guilliermondii, and Saccharomyces cereviasiae. The equivalent of 7.0 mg DM of yeasts was added and the presence of living cells in them was in the order of 10 7 cells . mL -1 . The parameters under study were: accumulated gas production, methane, ammoniacal nitrogen concentration and protozoan count. Only the I. orientalis and S. cereviasiae strains stimulated gas production and the inclusion of yeasts did not show an effect on the rest of the parameters under study. It is concluded that the species and strain of yeast have a decisive effect on the ruminal fermentation of fibrous substrates, which reaffirms the importance of selecting the appropriate strains for being used as an additive in ruminant diets, according to the food you want to use.