Yersinia enterocolitica is capable of growing in a broad range of temperatures from 4 to 45C. How this organism alters its membrane lipids in response to the change of growth temperature is very interesting. The fatty acids of membrane lipids of cells cultured at 5, 15, 25 and 37C were analyzed and the physical states of these membrane lipids were characterized. The major phospholipids of this bacterium were phosphatidylethanolamine, phosphatidylglycerol, cardiolipin, lysophosphatidylglycerol and lysophosphatidylethanolamine. No significant difference in phospholipid composition in response to culture temperatures was observed. It was reported in our previous paper that the major fatty acids of membrane phospholipids of Y. enterocolitica were C_15:0, C_16:0, C_16:1, cyclopropane C_17:0 and C_18:0. Some differences in the fatty acid composition were, however, observed with the change of culture temperature. When the culture temperature was raised, the saturated and cyclopropane fatty acids substantially increased and the unsaturated ones decreased. A reverse phenomenon was observed when culture temperature was lowered. From the viewpoints of membrane physical state, adaptational changes were analyzed using a nylon microcapsule method. Phase transition in membrane lipids of cells grown at each culture temperature took place in the range of about 5C below and about 10C above the culture temperature. It is, therefore, considered that Y. enterocolitica maintains its membrane rigidity and fluidity in response to growth temperature by changing the membrane fatty acid composition.