New water-soluble amphipatic hyperbranched organic polymers composed of a hydrophobic polystyrene core and a hydrophilic shell have been synthesized and characterized. An arborescent polystyrene core is first synthesized by the "graft-on-graft" technique based on the iterative grafting of end-functional polystyryllithium chains onto reactive poly(chloroethyl vinyl) ether backbones. Chain-extension of the external branches of the hyperbranched polystyrene is then achieved by living cationic polymerization of protected hydrophilic vinyl ethers. This yields polystyrene dendrigrafts surrounded by a dense poly(vinyl ether) shell that is made hydrophilic by deprotection of hydroxyl functions of vinyl ether units. The obtained nanometer-sized macromolecular structures are fully soluble in aqueous media and present unimodal and narrow size distribution with an average diameter of about 100 nm. The dimensions and shape of the individual macromolecules before and after the deprotection step were further investigated in solution by dynamic light scattering (DLS) and as isolated unimolecular deposits using atomic force microscopy and cryomicroscopy. The results obtained using both imaging techniques are in agreement with those obtained by DLS and provide additional information on the internal structure of the amphipatic dendrigrafts. Both the protected and the deprotected amphipatic polymers exhibit a complex internal organization constituted by distinct subdomains. These peculiar morphologies result from the internal segregation of chemically distinct macromolecular blocks that constitute the dendrigraft branches and associate with neighboring blocks to form separate phases.