We report the observation of confined modes in the mixed photon/exciton-like branches of exciton-polaritons propagating along the growth axis of a 700 nm-thick GaN film, deposited by Molecular Beam Epitaxy on bulk GaN substrates. The energies of the confined modes are in agreement with a simple modelling of quantized wavevectors in the thin layer, including the proper four-branch dispersion relation of polaritons in the wurtzite GaN. Such an observation results from the large coherence length of the exciton-polariton in the epilayer, testifying to the very good crystalline quality of the material. (C) 2001 Elsevier Science B.V. All rights reserved.