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We study the free exciton fine structure and the contribution of propagating excitations in the lower-polariton and upper-polariton branches (LPB and UPB) in the 2K fluorescence spectrum of GaN on Al2O3. The latter effect is observed for both A and B lines. A line-shape fitting of the photoluminescence was made using four Lorentzian functions. The distribution of polaritons in the UPB(A), LPB(B) and UPB(B) are found consistent with a Boltzmann electronic temperature of 29 K when exciting the fluorescence with a He-C laser at 325 nm. The longitudinal-transverse splittings can be extracted from the splittings between energies of dips in the PL bands at 3489.4 and 3497.8 meV and the values of the transverse excitons are smaller than 2 meV. (C) 1998 Elsevier Science B.V. All rights reserved.