An ultrasonic fuel injector for injecting a pressurized liquid fuel into the combustion chamber of an internal combustion engine that uses an overhead cam for actuating the injector, includes an injector body and an injector needle. The injector needle is disposed within the body and includes a magnetostrictive portion disposed in the region of the body defined by a ceramic wall, which is transparent to magnetic fields changing at ultrasonic frequencies. A wire coil is wound around the outside surface of the ceramic wall and connected to a source of electric power that is controlled to oscillate at ultrasonic frequencies during predetermined intervals of operation of the injector. A sensor is configured to signal when the overhead cam is actuating the injector to inject fuel into the combustion chamber of the engine. The sensor is connected to a control that is connected to the power source and is configured to operate same only when the overhead cam is actuating the injector to inject fuel into the combustion chamber of the engine. When the power source activates the oscillating magnetic field in the coil and applies same to the magnetostrictive portion of the needle, ultrasonic energy is applied to the pressurized liquid. A method involves retrofitting conventional injectors with needles having magnetostrictive portions and wound coils configured and disposed so as to subject the magnetostrictive portions of the needles to ultrasonically oscillating magnetic fields.