The typical power-time curves of six strains of Tetrahymena were determined by microcalorimetry at 28 A degrees C. Metabolic characteristics were obtained including metabolic rate constant (r), heat output in the log phase (Q (log)), total heat production (Q (T)), peak time, and measured and potential maximum power output (P (m) and P (max)). The power-time curves and metabolic characteristics show the differences among Tetrahymena strains. Euclidean distances and a clustering analysis diagram were obtained by metabolic properties. The smaller distances are between the four strains of Tetrahymena thermophila and the largest distance exists between Tetrahymena pyriformis GL and Tetrahymena pigmentosa HG2. These results are consistent with those obtained by first internal transcriptional spacer sequences. Therefore, the power-time curves and metabolic properties can be noticeable characteristics in distinguishing the different strains of Tetrahymena. And, the heat output in the log phase (Q (log)) is probably the most specific characteristic in distinguishing the different Tetrahymena strains and studying their relationships.