The previously proposed structures for the heterocyst glycolipids of the nitrogen-fixing cyanobacteriumAnabaena cylindrica have been revised. 1 3 C-enriched acetonides have been utilized for the determination of the relative stereochemistry of the 1,3-diol moiety in the aglycone of two glycolipids. The same glycolipids, which are thought to protect the nitrogenase from O 2 , have also been detected in A. torulosa.