Measurements of the phonon dispersion curves in naphthalene-d8, (deuteration >99%), taken at 77K are presented. The experiments were done on two crystals, using the triple-axis neutron spectrometers at the medium flux reactor, DR3 at Riso. Most of the external or lattice modes have been measured and assigned, but a small proportion of the modes were too weakly scattering to be positively identified. A least-squares minimisation procedure was used to improve the fit between the model and experiment. The agreement obtained is given by chi =2.7. These measurements show that on a high-flux reactor it will be possible to identify and measure all the lattice modes for crystals of the complexity of naphthalene.