The objective of this work was to evaluate a thermotolerant yeast Issatchenkia orientalis DY252 as a microbial feed additive for ruminants. In the present study, the influence of volatile fatty acids (VFA) and temperature on oxygen uptake rate by I. orientalis DY 252 was investigated. It was evident that the oxygen uptake rate was decreased gradually as the VFA concentrations increased in a range of 30 to 120 mM. Although the oxygen uptake rate was not greatly affected by temperature in the range 37 to 43°C, a maximum value of 0.45 mg O 2 /g cell/ min was obtained at 39°C. With regard to the oxygen uptake rate by yeast, viability was found to be less important than the metabolic activity of yeast.