During the aging of yeast culture, Kluyveromyces marxianus undergoes a number of changes in physiology and these changes play a significant role during fermentation. Aged stationary phase cells were found to contain more reactive oxygen species. Additionally, the level of oxidant is counteracted by the antioxidant defense system of the cells. Comparison of 3-day-old culture of K. marxianus with 45-day stationary phase culture represents an increased level of ROS inside the cells. Moreover, a decrease in glutathione content was observed over the set of the incubation period. The increased level of superoxide dismutase (SOD) and catalase also revealed that there is oxidative stress during the long period incubation of the stationary phase cells of K. marxianus. The actual phenomenon of aging in dairy yeast K. marxianus is a complex process, but the present study signifies that role of antioxidant defense system during aging in stationary phase cells of K. marxianus.