The study of the SDS-Formamide system by freeze-fracture electron microscopy revealed in addition to hexagonal phase H alpha and the cubic phase Q alpha the existence of a structure with flexible cylinders. This structure is located in the diagram of phases between hexagonal phase H alpha (p6m) and cubic phase Q alpha (Ia3d) and is characterised by defects (dislocations. disinclinaisons, focal domains, flexion and torsion boundaries). The hexagonal phase thus appears under two morphological varieties: a variety with rigid cylinders on the side low in surfactant and a variety with flexible cylinders on the side rich in surfactant. Any fractures present a network with square meshes revealing thus the cubic Q alpha (Ia3d). We think that the structure with flexible cylinders observed by electron microscopy is a stage to wards the transition H alpha -->Q alpha whose we will have the results.