A simple strategy for making nanoparticles by sonofragmentation of high-aspect-ratio 1D substrates is introduced. With common laboratory equipment, ultra-thin nanowires are fragmented into nanoparticles of size determined by the nanowire width, resulting within hours in monodisperse, crystalline nanoparticles of < 10 nm. This strategy is applicable to a diversity of semiconductor, oxide and metal nanowires.