Lipid analyses of the anaerobic bacterium Sphaerophorus ridiculosis revealed that 24.2% of the polar lipids are the alk-1'-enyl glyceryl ether (plasmalogen) form. The major polar lipids, phosphatidylethanolamine (67.5%), phosphatidylglycerol (11.2%), cardiolipin (12.0%), and lyso-phosphatidylethanolamine (9.3%), contained 26.3, 7.8, 5.2, and 13.4% plasmalogen, respectively.