In aqueous solution, pure DNA forms multiple liquid crystalline phases when the polymer concentration is increased. A structural analysis of the different phases and of their transitions is presented here in polarizing and in electron microscopy. The cholesteric phase presents usually periodic patterns with series of nested arches in electron microscopy. The defects of the structure can be analyzed with the resolution of the electron microscopy and compared to the data available in polarizing microscopy. We show how the germs of the more concentrated phase nucleate preferentially along some types of defects of the cholesteric phase. The concentrated phasg is columnar hexagonal and behaves as a lamellar structure. Each layer (25 to 40 Å thick) is composed of DNA molecules aligned in parallel. Numerous screw dislocations, either left or right-handed, can be seen in the structure. Besides, the evolution of the hexagonal textures, induced by a progressive dehydration of the preparation, can be followed with the resolution of the electron microscopy. Finally, the biological interest of the liquid crystalline structure of DNA is discussed.