The structure of medullary oil hyphae of twelve endolithic lichen species, belonging to different taxa and colonizing different habitats, was examined by light and electron microscopy. The chemical composition of lipids isolated from the oil hyphae and from two corresponding mycobionts grown in culture was determined. The oil hyphae of the various species appeared in different forms and contained large amounts of lipid in the form of oil globules. The hyphae of mycobionts isolated from two of the endoliths and grown in culture also contained large amounts of lipids. Triacylglycerol was the predominant lipid component in all the organisms examined. Hexadecanoic acid was the main saturated fatty acid; octadecenoic acid and octadecdienoic acid the predominant unsaturated fatty acids. Tetradecanoic, hexadecenoic, octadecanoic and octadectrienoic acids were also detected. The fatty acid distribution pattern appeared unaffected by the nature of substrate and climatic conditions. There is a certain similarity in the fatty acid composition in related species. 9 figures, 2 tables.