Ultrasound radiation has been used for the preparation of amorphous metals, amorphous alloys, amorphous metal oxides and amorphous metal nitrides. In all these cases, the cavitation phenomenon that leads to high temperatures and high pressures was utilized to synthesize the above-mentioned materials as powders having nanometer-sized particles. In this report, we describe the preparation of magnetic fluids formed by the application of ultrasound radiation. Three different systems in which stable magnetic fluids were formed are described. They represent (1) a cobalt colloidal solution in decalin stabilised by oleic acid, (2) a colloidal dispersion of amorphous metallic iron in a polymeric matrix, (3) a Fe2O3 colloidal solution in hexadecane stabilised by oleic acid.