The fatty acids of Yersinia enterocolitica were investigated by Abbas and Card. In their report, they stated that the major fatty acids were C16:0, C16:1, C17:0 and C18:1 and branched or cyclopropane side-chain fatty acids could not be detected. We, however, found a moderate amount of cyclopropane side-chain fatty acid. We could determine that this fatty acid was cis-9, 10-methylene hexadecanoic acid by gas-liquid chromatography, gas chromatography-mass spectrometry, silver-nitrate-treated TLC and PtO2 catalyzing hydrogenation method.