The lipid composition of the two closely related birchesBetula pubescensandBetula tortuosawas investigated. The total amount of lipids and the lipid composition was strikingly similar. The glycolipid content on a chlorophyll basis was high in both birches. A comparison of the. acyl groups revealed a small difference in 16:3 in monogalactosyl diacylglycerol, but the total amount of trienoic acids was similar. The acyl group composition in the nonplastid lipid phosphatidyl choline differed between the birches andB. tortuosahad a higher average number of double bonds. The observations on galactolipid and phospholipid could not explain the earlier stated higher tolerance ofB. tortuosato the freezing of its leaves