Self-assembling properties of an amphiphilic star block copolymer, 4-arm (PMMA-b-PAA)4 with poly(methyl methacrylate) core and poly(acrylic acid) shell, have been investigated in aqueous solutions. The stars have been observed to aggregate in a complex manner in aqueous solutions, depending on the presence or absence of salt. Screening of the electrostatic repulsion between the polyelectrolyte blocks by adding salt triggers the formation of worm-like micelles. The presence of worm-like micelles in saline solutions was revealed by Kratky analysis of the light scattering data. The coexistence of different micellar morphologies was visualized by direct imaging by cryoTEM. Due to the balance between the electrostatic and hydrophobic interactions semidilute solutions show interesting theological properties. Shear thinning behavior of salt-free solutions and their time-dependent gelation have been observed, thus indicating the strong tendency of the polymers to aggregate. (c) 2006 Elsevier Ltd. All rights reserved.