Contrast energy thresholds were measured for discriminating the direction of a drifting sinusoidal grating multiplied by an independently drifting space-time Gaussian (a generalized Gabor). We argue that the stimulus with the lowest contrast energy threshold identifies the receptive field of the most efficient linear motion filter. This optimal motion stimulus is found to be at 3 c/deg and 5 Hz, with a width and height of 0.44 deg and a duration of 0.133 sec, corresponding to spatial and temporal bandwidths of 1.1 and 2.5 octaves, respectively. The spectral receptive field is aligned more nearly to the Cartesian axes than to the velocity contour.