The heterocysts of limnetic nitrogen-fixing filamentous cyanobacteria contain unique glycolipids in their cell wall that create the distinctive gas impermeability of the heterocyst cell wall as well as serve as biomarker lipids for these microbes. It has been assumed that marine free-living and endosymbiotic cyanobacteria synthesize the same glycolipids although they have not been investigated in any detail. Here we report the glycolipid composition of several marine free-living heterocystous cyanobacteria as well as the heterocystous endosymbiont Richelia intracellularis found in the biogeochemically important diatoms Hemiaulus hauckii and Hemiaulus membranaceus. In the marine cyanobacteria Nostoc muscorum and Calothrix sp., we detected the same glycolipids as found in freshwater representatives of these genera. However, we did not detect these glycolipids in the Hemiaulus-Richelia association. Instead, we identified glycolipids which comprised a C₅ sugar, ribose, rather than the C₆ sugars normally encountered in glycolipids of free-living cyanobacteria. In addition, the glycolipids had slightly longer chain lengths (C₃₀ and C₃₂ versus C₂₆ and C₂₈) in the aglycone moiety. The different glycolipid composition of the marine endosymbotic heterocystous cyanobacteria compared to their free-living counterparts may be an adaptation to the high intracellular O₂ concentrations within their host. These glycolipids may provide unique tracers for the presence of these microbes in marine environments and permit exploration of the evolutionary origins of these symbioses.