Two kinds of lysoglycolipids, monogalactosyl 1-monoacylglycerol and digalactosyl 1-monoacylglycerol were generated in the cyanobacterium, Phormidium tenue, when it was stored at -20 degrees for more than 1 month. By comparison of the compositions of fatty acid residues between monogalactosyl 1-monoacylglycerol and monogalactosyl diacylglycerol, digalactosyl 1-monoacylglycerol and digalactosyl diacylglycerol, respectively, the 1-monoacylgalactolipids were presumed to be formed by regioselective deacylation at the sn-1 position with subsequent acyl-group migration from the sn-2 to the sn-1 position. In contrast, 1-monoacyl derivatives of sulphoquinovosyl diacylglycerol and phosphatidylglycerol, the other membrane lipids contained in cyanobacteria, were not formed.