The simple, one step synthesis of aqueous suspensions of amphiphilic nanoparticles is presented. Those particles are prepared in the batch heterophase polymerization of styrene or n-butyl acrylate, using a water-soluble poly(sodium acrylate) alkoxyamine macroinitiator. The nitroxide-mediated controlled growth of the hydrophobic block leads to the formation of poly(sodium acrylate)-b-polystyrene or poly(sodium acrylate)-b-poly(n-butyl acrylate) amphiphilic diblock copolymers, able to self-assemble in water simultaneously to the growth step. When the diblock copolymers become strongly asymmetrical, with a short poly(sodium acrylate) block and a long hydrophobic one, the formed hairy nanoparticles are analogous to amphiphilic diblock copolymer crew-cut micelles. © The Royal Society of Chemistry 2006.