In vitro gas production An interaction effect was observed (P=0.009) between treatment type and treatment dose for the asymptotic GP without interaction effects (P>0.05) for the rate of GP and the initial delay before gas production begins. In general, incubation of yeast CE improved (P=0.0007) the asymptotic GP compared to control and yeast LC. In both of CE and LC, the low and the intermediate doses were morelive cells (LC) or cells extract (CE) on in vitro gas production (GP) kinetics and ruminal fermentation parameters of a total mixed ration (TMR) consisting of commercial concentrate and alfalfa hay [1:1 dry matter (DM)] as a substrate was studied. The TMR was incubated with CE at 1, 2 and 4 mg/g or LC at 0.3, 0.6 and 0.9 mg/g DM for 96 h. Rumen GP was recorded after 6, 12, 19, 24, 48, 72 and 96 h of incubation. Interaction effects were observed (P<0.01) between treatment type and yeast dose for the asymptotic GP and methane (CH4) production. Incubation of yeast CE improved (P<0.01) the asymptotic GP compared to control and LC with greater effects (P<0.01) for the low and the intermediate doses. Yeast CE treatment was more effective (P<0.01) in GP than both of LC and control treatments with greater effect (P<0.01) for the low and the intermediate doses. Treatment type and yeast dose affected (P<0.01) CH4 production, metabolisable energy (ME), and short chain fatty acids (SCFA) without affecting in vitro DM degradability (IVDMD). Higher values (P<0.01) of CH4, ME, SCFA and IVDMD were observed for the yeast CE treatment. It could be concluded that adding yeast S. cerevisiae (CE and LC extract) improved GP and ruminal fermentation parameters, where CE at 0.3 and 0.6 mg/g DM was more effective than the yeast LC.