This study addressed the issue of how a stereo-specified surface influences the perceived two-dimensional orientation of a monocularly viewed line. In a series of three experiments, it was found that, in general, when a monocular line was embedded in a disparity field specifying a planar surface slanted about the horizontal axis, the tilt of the line depended on the slant of the surface in which it was embedded and on whether the line was presented in the left or in the right visual field. These results, predicted by perspective geometry, are compatible with the hypothesis that the monocular line is perceived as part of the stereo surface. Moreover, it was found that timing is a crucial factor in determining the strength of the effect. The effect was at a maximum when the monocular line and the stereo surface were presented together, with no prior presentation of the surface. The influence of the stereo surface on the monocular line was substantially reduced, however, when the surface alone preceded the combined presentation of the line and the surface.