The exciton binding energies are measured in GaN grown on A-plane sapphire. A value of 25Ϯ1 meV is found to be consistent with reflectance and photoreflectance experiments. This 25 meV value matches very well the results obtained on GaN epilayers grown on C-plane sapphire with strong biaxial compression, and seems to indicate that exciton energies in GaN epilayers are predominantly influenced by strain fields. From the in-plane anisotropy of optical response, we deduce a value of Ϫ2.4 eV for the C 5 deformation potential in GaN. ͓S0163-1829͑98͒03508-5͔

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