Microwave transitions and ground state rotational constants are reported for five newly synthesized deuterium isotopologues,Of cis-1,3,5-hexatriene (cHTE). These rotational constants along with those of the parent and the three C-13 species are used with vibration-rotation constants calculated from an MP2/cc-pVTZ model to derive an equilibrium structure. That structure is improved by the mixed estimation method. In this method, internal coordinates from good-quality quantum chemical calculations (with appropriate uncertainties) are fit simultaneously with moments of inertia of the full set of isotopologues. The new structure of cHTE is confirmed to be planar and is stabilized by an interaction between the hydrogen atoms H2 and H5, which form a bond and participate in a six-membered ring cHTE shows larger structural effects of pi-electron delocalization than does butadiene with the effects being magnified in the center of the molecule. Thus, strong structural evidence now exists for an increase in pi-electron delocalization as the polyene chain lengthens.