The calorific values of two protozoan species, Amoeba proteus and Tetrahymena pyriformis, were investigated for cells cultured at 20 °C, 15 °C, and 10 °C. Temperature had no effect on the energy content of Amoeba but differentiated between Tetrahymena cells cultured at 20 °C and 15 °C over those grown at 10 °C. This suggests that 10 °C is close to the organisms' lower thermal limit and that the cell has a different biochemical composition at this temperature.It is tentatively proposed that differences exist between the energy content of the two protozoan phyla represented. The calorific value of the ciliates may be close to 20.0 J mg−1 dry weight, whereas the sarcodines have a lower value approximating to 17.5 J mg−1 dry weight.