The effect of cationic surfactant on the compaction of DNA both in bulk solution and on polystyrene particles has been studied by dynamic light scattering. DNA undergoes a transition from a semiflexible coil to a more compact globule in aqueous solution as a consequence of cationic surfactant binding. Thus the hydrodynamic radius of DNA decreases from 107 nm to about 50 nm in the presence of cationic surfactant. Such compaction is also observed on DNA coated polystyrene particles. In this case, the hydrodynamic layer thickness of DNA coated particles is reduced from 36 to 5 nm in the presence of cetyltrimethylammonium bromide.