Complexation between DNA and cationic/nonionic micelles of dodecyldimethylamine oxide (DMDAO) at various ionic strengths (I) was studied by monitoring turbidity or dynamic light scattering as a function of pH at various ionic strengths (I). Complexation takes place at a critical degree of micelle protonation (beta (c)) and is therefore primarily controlled by the ratio of cationic:nonionic surfactant, not by the ratio of surfactant:DNA. beta (c) corresponds to a critical micelle surface charge density (sigma (c)), which increases nearly linearly with I. The dependence of beta (c) on I displays a discontinuity when the DMDAO micelle shape changes from spherical to rodlike. The proximity of the complexation boundary to the coil/coil-globule coexistence boundary reported from fluorescence microscopy(1) indicates that the DNA-micelle complexation observed by scattering also corresponds to micelle-induced collapse of DNA.