Weak phosphorescence of a stiff cis-stilbene analogue, 5H-dibenzo[a,d]cycloheptene single crystal, has been observed at 77 K. The energy of the T-1-S-0 transition is 222 kJ mol(-1), and the lifetime of the lowest excited triplet state is 14 ms. For interpreting the results, the ground state geometry of the molecule has been determined by the X-ray crystallography. The torsion angles around C(ethylene)=C(ethylene) and C(ethylene)-C(phenyl) are 0 degrees and 32.6 degrees, respectively. The results are discussed and compared with those of cis-stilbene.