Transient polarized resonance Raman spectra of naphthalene and its fully deuterated species in the lowest excited triplet state were measured. An analysis of the depolarization ratios showed that all the observed bands were assignable to the ag vibrational modes. Matrix-isolation infrared spectra of the excited state [J. Mel. Struct. 1999, 475, 253] were remeasured to observe further bands with weak intensities. The DFT (density functional theory) calculation was also performed to confirm the vibrational assignments, where the 6-31G* basis set was used to optimize the geometrical structure. The DFT calculation is found to be useful to predict the vibrational wavenumbers of the lowest excited triplet state as well as the ground singlet state of this molecule.