The magnetoelec. effect of Cr2O3 single crystals was studied in magnetic fields up to 20 T in the range of liq. He to room temp. In the antiferromagnetic phase for low magnetic fields the known results for the magnetoelec. effect below the Neel temp. were reproduced. In the spin-flop phase, for magnetic fields above a crit. field (H>Hcrit .simeq. 10 T), the magnetic field induced elec. polarization was measured along all three crystallog. directions. Magnetoelec. effect, linear in the magnetic field was found for the three orientations leading to the conclusion that the magnetic ground state symmetry of the spin-flop phase is either .hivin.1' or 2/m', where 2'/m can be excluded.