Gas-phase electron-diffraction data obtained at 351 K, together with ab initio molecular orbital calculations (HF/6-31G*) have been used to determine the structure and conformation of hexachloropropene, Cl3C-CCl=CCl2. The experimental-data may be fitted using a model with a conformer where one carbon-chlorine bond is anti to the carbon-carbon double bond. No indications of a second conformer were observed, but small amounts (<10%) of a form where C-Cl is syn to C=C cannot be excluded. The results obtained from the combined ED/ab initio investigation for the bond distances (r(g)) and valence angles (angle(a)) are r(C=C) = 1.350(13) Angstrom, r(C-C) 1.536(17) Angstrom, [r(C-Cl)] = 1.748(3) Angstrom, angle C-C=C = 124.4(11)degrees, angle C-1=C-2-Cl = 117.7(21)degrees, [angle C-2-C-1-Cl] = 122.6(11)degrees, angle C-2-C-3-Cl-8 = 114.6(26)degrees, angle C-2-C-3-Cl-8 = 109.9(14)degrees, angle Cl-8-C-3-Cl-9 = 109.9 degrees (assumed). Error limits are given as 2 sigma where a include estimates of uncertainties in voltage/height measurements and correlation in the experimental data. The results are compared with those from related molecules and with results from ab initio calculations.