The effect of cetyltrimethylammonium bromide (CTAB), a cationic surfactant, was investigated on the aggregated form of DNA molecules in water at a given temperature. CTAB caused changes in the aggregated form of DNA molecules from loosely packed spherical to rodlike through toroidal one with increasing concentration below the critical micelle concentration (CMC). The change was suggested to arise from the ion complex formation between CTAB and DNA molecules due to the electrostatic interaction, thereby reducing the surface charge and solubility in water of the latter.