The lipid composition of Prochlorothrix hollandica , a photosynthetic prokaryote that contains chlorophylls a and b , was found to be similar to that of cyanobacteria and that of another member of the prochlorophyta, namely, Prochloron sp. Monogalactosyl diacylglycerol was the most abundant class of lipid and monoglucosyl diacylglycerol was present only in trace amounts. Digalactosyl diacylglycerol, sulfoquinovosyl diacylglycerol and phosphatidylglycerol were present at intermediate levels. Individual lipid classes revealed great diversity in terms of their fatty acid composition. Monogalactosyl diacylglycerol and digalactosyl diacylglycerol possessed higher levels of unsaturated fatty acids than did sulfoquinovosyl diacylglycerol and phosphatidylglycerol. J49- cis -Hexadecadienoic acid was bound to monogalactosyl diacylglycerol and digalactosyl diacylglycerol. The distribution of fatty acids at sn -positions of the glycerol moiety was unique in that C 14 fatty acids were located mainly at the sn -1 position while C 16 fatty acids were located at the sn -2 position. δ4- cis -Hexadecenoic and δ49- cis -hexadecadienoic acids were esterified to the sn -2 positions in each of the lipid classes, whereas δ9- cis -hexadecenoic acid was esterified to both sn -1 and sn -2 positions.