Recent developments in touch and display technologies made it possible to integrate touch-sensitive surfaces into stereoscopic three-dimensional (3D) displays. Although this combination provides a compelling user experience, interaction with stereoscopically displayed objects poses some fundamental challenges. If a user aims to select a 3D object, each eye sees a different perspective of the same scene. This results in two distinct projections on the display surface, which raises the question where users would touch in 3D or on the two-dimensional (2D) surface to indicate the selection. In this paper we analyze the relation between the 3D positions of stereoscopically displayed objects and the on- as well as off-surface touch areas. The results show that 2D touch interaction works better close to the screen but also that 3D interaction is more suitable beyond 10cm from the screen. Finally, we discuss implications for the development of future touch-sensitive interfaces with stereoscopic display.