The respiratory energy losses ofStentor coeruleus Ehrenberg are described in relation to cell size at 15°C and 20°C. Values forb of 0.6736 at 20°C and 0.6013 at 15°C were found. The rate of increase in metabolism between 15-20°C represents aQ 10 of between 2.02-2.89. A maximum rate of oxygen uptake in the region of 3.5x10-4μl/hr for ciliates of over 1100 ng dried weight was recorded at 20°C and 1.5 × 10-4 - 1.8 × 10-4 μl/hr for ciliates weighing 500-860ng at 15°C. The results are discussed in relation to other protozoa.