We investigated the vibrational energies in the 2p (2)A(2) " and 3d E-2 " states of the triatomic deuterium molecule D-3. The experiments were performed using a fast neutral beam photoionization spectrometer recently developed at Freiburg. A depletion type optical double-resonance scheme using two pulsed dye lasers was applied, The measured vibrational frequencies of the 2p (2)A(2) " state of D-3 are compared to those of H-3 and to theoretical values calculated from an ab initio potential energy surface. The data give insight into the importance of the coupling between the valence electron and the ion core.