The author studied the membrane lipid and fatty acid compositions of Yersinia enterocolitica. Furthermore, the author studied the adaptation of the membrane to changes in culture temperatures, and characterized the physical state of membrane lipid by nylon microcapsule method developed by Okahata and Nagamachi. Phosphatidylglycerol, phosphatidylethanolamine and cardiolipin were the main phospholipids of the membrane. The ratio of these phospholipid compositions was not changed by culture temperature. Fatty acids were found to be C(14:0), C(15:0), C(16:0), C(16:1), C(17:0), C(18:0), C(18:1) and cyclopropane C(17:0). The last was identified by gaschromatography-massspectrometry and hydrogenation. In 37°C cultures, saturated fatty acids were increased and unsaturated fatty acids were decreased in comparison with those from 25°C cultures. Cyclopropane C(17:0) was also increased in 37°C cultures. These changes appear to be necessary for maintaining membrane stability at high temperatures. Using NaCI-release from 0.2M NaCI-entrapped nylon microcapsules coated with the membrane lipids as an indicator, it was found that phase transition of lipid membranes from 25°C cultures and 37°C cultures occurred approximately at 30°C and 45°C, respectively. The differences in the phase transition temperature seem to correlate with the adaptive changes in fatty acid composition by culture temperatures.