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Results of recent electron diffraction studies of several halo-substituted propenes have been interpreted in terms of a deviation from $C8$ symmetry which destroys the coplanarity of the bonds about the C=C bond. Such a result is surprising in view of the well-documented planarity of the ethylenic skeleton in many olefin derivatives. In principle, the rotational spectra of such molecules can yield much more precise information about the coplanarity of atoms than can the diffraction data. We have analyzed the microwave spectrum of hexafluoropropene, $CF2=CF-CF3$, for which a twist of $\sim 40\circ$ about the $C=C$ bond has been reported. Unfortunately, the most conclusive tests of $C8$ symmetry, i.e. direct determination of $\mu a,\mu b$ and $\mu c$ or isotopic substitution at one or more of the carbons, are difficult to carry out. Nevertheless, the measure of $\Sigma m1c12$ afforded by the observed moments of inertia gives strong support for a “planar” or $C8$ geometry, in contrast to the diffraction results. The quantity $Ia+Ib-Ic$, which depends only on the moment of the $CF3$ group if a planar skeleton is assumed, is observed to be 88.256 amu \AA$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$. This agrees well with the value calculated from structural data on typical $CF3$ groups. Thus, any appreciable departure from C, symmetry would require the $CF3$ top in hexafluoropropene to have a highly distorted shape. The observed rotational constants of the ground state are 2557.9, 1255.0, and 987.1 MHz. Several excited states of the $CF3$ torsion have also been assigned. Relative intensity measurements allow a rough estimate of the barrier to $CF3$ rotation.