Abstract: In this study, we initially compared the cold tolerance of the Genetically Improved Farmed Tilapia(GIFT) strain of Nile tilapia(Oreochromis niloticus) and the paradise fish(Macropodus opercularis) with a series of scientific indices, such as the temperature at death(TAD), the cumulative degree hours(CDH) and the survival curves. Next, we systematically studied the effects of low temperatures on a series of plasma membrane-related physiological functions, such as membrane fluidity, endocytosis and the activity of the membrane protein Na+, K+ -ATPase in both species. The results showed that the paradise fish had a better cold tolerance than did the GIFT. The plasma membrane-related physiological functions were all sensitive in low temperatures of both fishes. However, these sensitivities were different between the hardy species(paradise fish) and the non-hardy species(GIFT). The patterns and tendencies concerning sensitivity changes during varied temperatures were consistent in each species. All these interspecific differences and intraspecific consistence highlighted the importance of the membrane fluidity during the cold acclimation. These results suggest that the plasma membrane, particularly membrane fluidity, may play an important role in cold acclimation of the two fish species.