The γ-ray flux in the Fréjus underground laboratory has been studied using an NaI detector surrounded by different diagonostic shields. Below 4 MeV, the spectrum is dominated by radioactivities in the surrounding materials and rocks. Between 4 and 6 MeV, the shape of the spectrum is well explained by U, Th and daughters, which are internal contaminations in the NaI crystal. Between 6 and 10 MeV, the γ-ray flux is strongly correlated with neutron captures in the surrounding materials. Finally, the γ-ray flux above 10 MeV falls off and is related to the very weak cosmic muon flux via muon bremsstrahlung.