Trigalactosyl diglyceride has been isolated from tubers of potato (Solanum tuberosum) by a combination of chromatographic methods. This galactolipid, which constitutes approximately 1% by weight of the total lipids, was characterized by analysis of the intact lipid and its deacylation product. The fatty acids:glycerol:galactose molar proportions were shown to be close to 2:1:3. Evidence was obtained that suggests that trigalactosyl diglyceride is a higher homologue of mono- and di-galactosyl diglycerides and contains an additional d-galactopyranosyl moiety that is linked alpha-(1-->6) to the terminal galactose unit of digalactosyl diglyceride.