A three-dimensional X-ray analysis of the crystal and molecular structure of 1,8-bisdehydro- [14]annulene, C
14
H
10
, is described. Carbon-carbon and carbon-hydrogen bond lengths have average estimated standard deviations of 0.009 and 0.06 Ǻ respectively and are discussed in the light of molecular orbital predictions of the electron density in the molecule; the root mean square agreement between observed and calculated carbon-carbon bond lengths is better than 0.01 Ǻ. Distortions of the carbon bond angles from strict trigonal and digonal symmetry are attributed to intramolecular steric interactions between hydrogen atoms and carbon or other hydrogen atoms. An analysis of the molecular vibrations in the lattice shows the three r.m.s. translational amplitudes about the molecular symmetry axes to be 0.18, 0.19, 0.18 Ǻ, the three corresponding r.m.s. librational amplitudes being 2.9, 2.2 and 3.6°. The ‘difference' density syntheses (σ = 0.06 e/Ǻ
3
) are discussed and the crystal structure is related to anthracene and phenanthrene, isomers of bisdehydro-[14]annulene.
