Capillary gas chromatography-mass spectrometry (GC-MS) analysis of the sterol ester fatty acid methyl esters of the crayfish hepatopancreas revealed the presence of at least 30 kinds of unusual furan fatty acids (F acids), which accounted for 28.49% of the total sterol ester fatty acids. On the other hand, only small amounts were found in triacylglycerols (0.5%) and phospholipids (0.7%). Among the F acids, 17 acids were the hitherto unknown ones, the major component being 12,15-epoxy-13,14-dimethyleicosa-12,14-dienoic acid (F6) (41.83% of the total F acids). These novel acids possessed chain lengths of C12 to C22, with the largest concentration at C20 (45.38%), C18 (41.97%) and C16 (10.35%). Odd-numbered F acids also were found, though in a very small amount (0.4%). In the living things other than the crayfish, the longest chain F acid (C24) was detected in the sterol ester of the carp hepatopancreas. The bullfrog, an amphibian, and the turtle, a reptilian, were found to have F acids as well in their livers.Olefinic furan fatty acids, which were detected by GC-MS, were found to have resulted during the analytical process from cyclodehydration of the diketo-ene formed by autoxidation of the F acids.