Amphoteric polyurethane (APU) samples used in this paper were composed of hydrophobic soft segments and pendent -COOH and -CH2N(CH3)(2) groups on the hard segments, which present the properties of both amphoteric polyelectrolytes and amphiphilic block copolymers. APU macromolecules can self-assemble into micelles in acidic and basic aqueous media by hydrophobic/hydrophilic interaction. The self-assembly behavior of APU in acidic and basic media was studied by transmission electron microscopy and light scattering methods. The spherical and hollow micelles of APU were observed respectively in acidic and basic aqueous media. The results indicate that the size and size distribution of APU self-assembly micelles largely depend on the ratio of -COOH to -CH2N(CH3)(2) groups, density of ionizable groups, concentration of APU, and types of acid and base in the media. (C) 2003 Elsevier Inc. All rights reserved.