Abstract 1. 1. The dorsal aorta of carp ( Cyprinus carpio L.) was cannulated and fish were kept in thermostated aquaria at 5°C for 1 week, after that the water temperature was gradually increased to 25°C at the rate of 0.5°C/hr. 2. 2. Blood was withdrawn at each 5°C interval after fish were acclimated for 3 days to determine the changes in the lipid peroxidation, fatty acid patterns of phospholipids and Na +-K +-ATPase activity. 3. 3. Fluidity of erythrocytes plasma membrane sensitively followed the changes in water/body temperature in such a way that it decreased with wanning and increased with cooling. 4. 4. The level of long chain polyunsaturated fatty acids composition in phospholipids (PLs), was much higher in the cold adapted fish than in warm adapted ones. 5. 5. Phospholipids of the warm adapted fish were characterized by a higher level of saturated fatty acids especially palmitic and stearic acids than those of cold adapted fish. Lipid peroxidation, was significantly increased in cold adapted rather than in the warm adapted fishes. 6. 6. Correlating the acyl chain indexes of phospholipids with lipid peroxidation provided a non marked interrelationship, except for the 20: 3(n-9): 20: 5(n-3) ratio, which showed an inverse linear correlation to the rate of lipid peroxidation. 7. 7. Na +-K +-ATPase: in cold water the carp started to preserve the elevated ATPase activity while higher temperature caused a decrease in the enzyme activity.