The vibronic structure of the phosphorescence spectrum of naphthalene is calculated from ab initio and semiempirical geometries and force-fields of the S-0 and T-1 states. An excellent agreement with the experiment is obtained. The final triplet state in the triplet --> triplet absorption spectrum is identified as 3 B-3(3g) state and the vibronic structure of the 1 B-3(1u) --> 3 B-3(3g) transition is found in a good agreement with the experimental spectrum. The wavefunction of the 3 B-3(3g) state is dominated by the same electronic configurations as that of the lowest tripler of the same symmetry. (C) 1997 American Institute of Physics.