Novel magnetic nanocomposites (see Figure) are obtained by the self-assembly in water of polypeptide-based diblock copolymers of polybutadiene-block-poly(glutamic acid) combined with hydrophobically modified gamma-Fe2O3 nanoparticles. These hybrid supramolecular objects are either filled micelles (three dimensional) or hollow vesicles with a magnetic membrane (two dimensional), the deformation of which, under an applied magnetic field, has been evidenced.