We studied the interaction between individual long genomic DNA molecules and cationic lipid assemblies. The assembly of cationic lipid molecules into small unilamellar vesicles (SUVs) of about 50 nm diameter led to the compaction of DNA whereas the addition of a neutral surfactant resulted in the disruption of SUVs and the unfolding of DNA, This reversible process does not require any chemical reaction or change in the ionic strength of the solution. It was applied to switch DNA repeatedly between a compact and an unfolded conformation in a dynamic manner.