Abstract: This paper describes a comparative study of the activities of the mitochondria ATP ase of the liver cells in Ctenopharyngodon idella. Leuciscus waleckii and Cirrhina molitorella, acclimated to low and high temperatures. Observations were made on the effect of Tween-80 treatment of the mitochondria on the transition temperature in the Arrhenius plot for the activation energy of the mitochondria ATPase. Compared with fish acclimated at higher temperatures, fish acclimated at lower temperatures showed lower transition temperatures in the Arrhenius plots for the activation energy of their mitochondria ATPase. Treatment with Tween80 resulted in decreased transition temperatures. It was suggested that the magnitude of reduction in the transition temperature when the fish are acclimated to a lower temperature is correlated with the fish's capacity of cold-resistence. The transition temperature is related to tolerance to low temperature.