The absorption spectrum of the open-shell molecular ion SD+in itsX3Σ-ground state has been observed by the mid-infrared L.M.R. technique for the first time. Pure vibrational rotational transitions were detected with 53 CO-laser-lines and 120 Zeeman components were assigned to four vibrational rotational bands of the SD+ground state. The spectroscopic constants for the ground state were improved by at least an order of magnitude.