The effect of oxygen on anaerobic protozoa was studied in anaerobic batch reactors inoculated with sludge and protozoa cultures. Among the protozoa genera, Metopus, Brachonella, Plagiopyla, Trepomonas, and Vanella were more sensitive to oxygen compared to other genera. Protozoa genera Menoidium, Rhynchomonas, Cyclidium, Spathidium, and Amoeba were found to survive under aerobic conditions, and the growth rate was slightly higher or similar to anaerobic condition. O2 tension resulted in the loss of free and endosymbiotic methanogens in anaerobic system, while methanogens were observed inside the protozoan cysts. Survival of anaerobic protozoa declined considerably when the O2 tension exceeded 1% atm. sat. and showed chemosensory behavior in response to O2 exposure. Superoxide dismutase activity was detected in survived protozoa cells under O2 tension. Facultative anaerobic protozoa with SOD activity can provide a mechanism to overcome possible occurrence of oxygen toxicity in the treatment of wastewater in anaerobic reactor.