We propose a novel all-optical gate switch based on a frequency shift accompanied by the semiconductor band-filling effect. This all-optical switch is very simple in structure, yet is capable of ultra-high-speed and high-efficiency switching. The switching characteristics are numerically investigated with a rate equation model for nonlinear phase shift. Then, the principle of operation is experimentally verified by measuring the spectral and time-domain properties of the switch. Its switching time is nearly the same as the duration (600 fs in the present experiment) of a control pulse. ©1997 American Institute of Physics.