An ellipse rotating in the image plane can produce several different percepts. The two-dimensional (2D) percepts are either a rotating rigid ellipse or a constantly deforming non-rigid ellipse. The 3D percept is a rotating rigid circular disk that is tilted relative to the image plane. Stimuli that generate 3D percepts based on purely 2D rotational motion are known as stereokinetic stimuli. We examined the 3D percepts generated by the rotating ellipse stimulus. In theory, the motion of the 3D percept cannot be reliably inferred based on the 2D stimulus. When we quantitatively estimated observers' perceived motion, however, we found that the perceived motion was nearly identical across observes. These results suggest that all observers had similar 3D percepts. We assumed that given the 2D rotating ellipse stimulus the visual system generates a rigid 3D percept that is as slow and smooth as possible. The percepts predicted by these assumptions closely matched the experimental data. These findings suggest that perceptual ambiguity in stereokinetic stimuli is resolved using slow and smooth motion assumptions.