Effects of dark treatment and N_1-dichlorophenyl-N_3-dimethylurea (DCMU) on the desaturation of galactolipids of Dunaliella salina were investigated to see whether light-driven photosynthetic electron transport is involved in in vivo desaturation of galactolipids. The incorporattion of radioactive fatty acid precursors ([^14C]lauric acid) into galactolipids, mainly composed of prokaryotic molecular species, was most affected among different polar lipid classes by both treatments. The analysis of specific radioactivities of individual galactolipid molecular species revealed that their synthesis was greatly inhibited by the treatments except for eukaryotic molecular species, 18:3/18:3 digalactosyldiacylglycerol, whose desaturation occurs in endoplasmic reticulum.