The ESR spectra of the high Tc superconductor YBa2Cu3O7 show the structure corresponding to the Cu2+ (3d9, S= 1 2, I= 3 2) ions in an anisotropic environment and in the presence of rapid oscillating distortions. © 1987.64710431045Bednorz, Muller, Possible highT c superconductivity in the Ba?La?Cu?O system (1986) Zeitschrift f�r Physik B Condensed Matter, 69 B, p. 189Wu, Asburn, Torny, Hor, Meng, Gao, Huang, Chu, (1987) Phys. Rev. Lett., 58, p. 908Siegrist, Sunshine, Murphy, Cava, Zahurak, (1987) Phys. Rev., 35 B, p. 7137Hazen, Finger, Angel, Prewitt, Ross, Mao, Hadidiacos, Chu, (1987) Phys. Rev., 35 B, p. 7238RETTORI C., DAVIDOV D., BELAISH I. & FELNER I., submitted to Phys. Rev. BKitazawa, Atake, Ishii, Sato, Takagi, Uchida, Saito, Tanaka, (1987) Jap. Journal of Applied Phys. Letters, 26, p. 748Orton, Auzins, Griffiths, Wertz, (1961) Proc. Phys. Soc. (London), 78, p. 554Coffman, Jahn-Teller Effect in the EPR Spectrum of Cu++: MgO at 1.2°K (1968) The Journal of Chemical Physics, 48, p. 609Che, McEwan, Wenger, Logothetis, Determination of charge carriers in superconducting La-Ba-Cu-O by thermoelectric measurements (1987) Physical Review B, 35 B, p. 7124BIANCONI A., CLOZZA A., CONGIU CASTELLANO A., DELLA LONGA S., DE SANTIS M., DI CICCO A., GARG K., DELOGU P., GARGANO A., GIORGI R., LAGARDE P., FLANK A.M., & MARCELLI A. (to be published)Hoeck, Nickisch, Thomas, (1983) Helvetica Physica Acta, 56, p. 237Chakraverty, (1979) Journal de Physique (Letters), 40, p. 99Kamimura, Cooperative Bipolaron Tunneling in HighTcCopper Oxide Compounds and Superconductivity (1987) Japanese Journal of Applied Physics, 26, p. L627