The phosphoglyceride phosphatidyl-O-[N(2-hydroxyethyl)glycine] (PHEG) has been isolated from Fucus serratus L. (Fucales, Phaeophyceae) and its structure determined by IR, NMR, MS and chemical data of the intact lipid (formerly called PX), its lyso compound and the deacylated lipid. The fatty acid composition was about 82 mol% 20: 4, 9% 20: 5 and 6% 20: 0 acids in both the sn-1 and the sn-2 positions of the glycerol part. PHEG was present in 30 different species representing the 16 orders of brown algae in amounts of 8–25 mol% of total phospholipids, and is suggested to be a compulsory lipid constituent of brown algae.