We report electron paramagnetic resonance experiments in single crystals of the high Tc superconductor YBa2Cu3O7-x. No EPR signal is observed in the spectra of the properly annealed crystals with Tc≅90K. However, we observe a strong, almost isotropic Cu2+ signal in the spectra of the as-grown (unannealed) single crystals with Tc≅40K. The observed signal has a slightly anisotropic g-factor which becomes totally isotropic at T≅90K. The linewidth, dominated by the hyperfine interaction, remains anisotropic at all temperatures, showing that the signal originates from a site which does not have real cubic symmetry. © 1988.