In the absence of samples suitable for transmission measurements, photoluminescence excitation spectra (PLE) have been found useful in the evaluation of detailed information about the lowest direct-absorption edge of GaN. In this work the results of PLE measurements are combined with data on reflection and luminescence in the intrinsic region to determine the positions of A-, B-, and C-exciton ground-state transition energies and the lowest band gap. Neglecting polariton effects, the value of the A-exciton ground-state transition energy is determined as being EAx=3.4751±0.0005 eV at 1.6 K from combined PLE and emission spectra. The corresponding values for B and C exciton transitions are found to be EBx=3.4815±0.001 eV and ECx=3.493±0.005 eV from PLE spectra. The lowest band gap is determined to be EgA=3.503-0.002+0.005 eV at 1.6 K, which fixes the ground-state A-exciton binding energy as EB(A)=28-3+6 meV, in good agreement with the effective-mass value. The temperature dependence of the band gap could also be accurately measured in PLE spectra and can be described by an expression EgA=[3.503+(5.08×10-4T2)(T-996)] eV for T<295 K, with an estimated relative uncertainty of ± 0.002 eV. © 1974 The American Physical Society.