A fuel supply system for mixture-compressing internal combustion engine with externally-supplied ignition is proposed, which serves to form the most optimal possible fuel-air mixture. The fuel supply system includes a primary mixture formation unit disposed in the intake manifold and at least one ultrasonic atomizer nozzle in the intake manifold, for the purpose of correcting the fuel-air mixture by means of supplementary fuel, downstream of the primary mixture unit or in each individual intake tube. The ultrasonic atomizer nozzles are triggered, with the interposition of an electronic control device, in accordance with operating characteristics of the engine, as well as the exhaust composition, and they permit the supply of finely prepared supplementary fuel for the purpose of regulating the fuel-air mixture to a predetermined .lambda. value; of compensating for non-uniform distribution, and of enrichment of the mixture in the case of warm-up, full load and acceleration, as well as of idling regulation.