CdSe hollow spherical assemblies composed of 5 nm nanoparticles have been synthesized sonochemically. During the process, amorphous Cd(OH)(2), which acts as the in-situ template, directs the growth of primary CdSe nanoparticles on its surface and their assembly into hollow spherical structures. The Figure is a schematic of the proposed mechanism for the formation of the hollow chalcogenide spheres.