A procedure has been described for obtaining accurate measurements of the rate of oxygen-consumption of Protozoa. This procedure involves the use of Warburg manometers and concentrated suspensions of organisms made up from bacteria-free cultures. The rate of oxygen-uptake by the holotrichous ciliate Colpidium campylum has been determined. The average of 36 measurements gave a value of 112.5 mm.3 Ō2/hr./million for the rate at 19.8° C. in the medium in which the infusoria had been growing for 48 hours. The rate remained constant over a period of more than four hours. The standard deviation of the measurements was 0.0363.