X-ray single crystal analysis of 1, 5 , 9-trans, trans, trans-cyclododecatriene (CDT) has been performed by multiple film Weissenberg equi-inclination tecniques, the crystal being held under a cooled nitrogen stream. The monoclinic unit cell has the following constants: a = 14.20 ± 0.06; b = 8.41 ± 0.03; c = 9.37 ± 0.04 Å; ß = 109.1 ± 0.50; Z = 4. The approximate orientation of the molecule, assumed as corresponding to $\mathbf{D}_{3}$ symmetry, and the choice of the space group, have been obtained by trial methods; resulting space group is C2/c. The structural refinement has been carried on through 4 cycles of three-dimensional least-squares method (hkl reflections with l = 0, 1, 2, 3, 4); the final molecular conformation is in good agreement with the idealized $\mathbf{D}_{3}$ symmetry, and is very close to the model “a priori” assumed on the basis of minimum intramolecular energy requirements. Bond lengths and angles are also close to the expected values. Both the molecular conformation and the crystal packing are very similar to those found for the Ni-CDT complex [7].