An air intake system for an internal combustion engine wherein the intake port to a combustion cylinder is connected to a trunk portion of the intake system which contains the throttle valve. A high speed intake passage of a relatively short length and a slow speed intake passage of a relatively long length are both connected to the trunk portion with a control valve in the high speed intake passage. The control valve is opened for high speed operation of the engine and closed for slow speed operation to operatively select between the two intake passages. In modified embodiments the degree of opening of the control valve is modulated in proportion to the r.p.m. of the engine.