The spatial parameters underlying a novel illusion of relative motion are characterized. A simple stimulus composed of two sine-wave gratings was sufficient to generate the illusion. We measured the response of subjects to rapid, small-amplitude oscillations of this stimulus behind a fixation point. The effect was clearly strongest for acute angles between the gratings, but only when spatial frequency was between 6 and 11 cpd. We surmise that activity in the grating cells of the primate visual cortex (von der Heydt, Peterhans, & Dursteler, 1992) might be the cause of the illusion. The illusion is potentially an important tool in understanding how higher cortical areas combine disparate motion signals.