The velocity‐selective characteristic of the interaction between a monochromatic radiation field and a Doppler‐broadened molecular transition is utilized to obtain a narrow saturation resonance for molecules with a given velocity component along the propagation direction of the radiation field. The width of the observed resonance gives the dependence of collision broadening on molecular velocity. The effect is observed in an infrared transition in NH3for self‐broadening and foreign gas broadening by Xe.