The nine-channel 337 μm interferometer/polarimeter on the TEXTOR tokamak has been used to detect and control the horizontal plasma position in two different ways. (1) The phase shift between two vertical probing beams located at a distance of x/a = ±0.6 from the vessel centre yields the difference of electron line densities along these trajectories and indicates displacements or asymmetries of the density profile. (2) Faraday rotation measurements on a probing beam passing vertically through the vessel centre allow horizontal shifts of the magnetic flux surfaces to be detected. Both the phase shift and Faraday rotation signals have been applied as control quantities to the TEXTOR feedback system, resulting in good positional stability for discharge durations of more than 2.5 seconds.