The microwave rotational spectrum of ([1,2-D-2]benzene)chromium tricarbonyl was measured in the 4-12 GHz range using a Flygare-Balle-type spectrometer. This spectrum contains transitions due to two different structural isomers of this complex. These results are interpreted in terms of a reduction of the symmetry of benzene to C-3v due to interactions with the Cr(CO)(3) moiety. One structural isomer (isomer E) occurs when the deuterium atoms are at the ends of a "long" C-C bond, the other (isomer S), when the deuterium atoms are at the ends of a "short" C-C bond. The data indicate a difference of 0.016 Angstrom in adjacent benzene C-C bond lengths in this complex.