We investigate the electronic properties of copper ions substituting zinc ions in diaqua(L-aspartato)Zn(II) hydrate (ZnC4H5O4N . 3H(2)O), to be called Zn(L-asp). In this compound, the Zn ions are in an octahedral coordination with a small distortion in the equatorial plane and the aspartate ion acts as a tridentate ligand. EPR spectra of single crystals of Zn(L-asp) doped with Cu-63(II) were obtained at 9.8 GHz between 77 and 300 K, with the magnetic field applied in the crystallographic planes ab, bc, and ca. The g-factor and Cu-63 hyperfine coupling along the crystal axes display large temperature variations. From the single-crystal data we obtained the principal values of these tensors: g(1) = 2.0377(5) [2.028(1)], g(2) = 2.1701(5) [2.144(2)], g(3) = 2.3127(4) [2.351(2)], A(Cu)(1) = 61.5(4) x 10(-4) cm(-1) [58.0(3) x 10(-4) cm(-1)], A(Cu)(2) = 20.5(1) x 10(-4) cm(-1) [3.6(3) x 10(-4) cm(-1)], A(Cu)(3) = 87.8(2) x 10(-4) cm(-1) [114.5(4) x 10(-4) cm(-1)], at 300 K [77 K]. At 77 K, the hyperfine coupling with the N-14 ligand is resolved and the principal values of the A(N) tensor are A(N)(I) = 11(2) x 10(-)4 cm(-1), A(N)(2) = 11(2) x 10(-4) cm(-1), A(N)(3) = 8(2) x 10(-4) cm(-1). EPR data in powder samples at X-band and Q-band were compatible with those values. From the Q-band powder spectrum at 5 K we get g(1) = 2.03(1), g(2) = 2.12(3), g(3) = 2.36(3), A(Cu)(1) = 55(1) x 10(-4) cm(-1) \A(Cu)(2)\ = 4(2) x 10(-4) cm(-1), A(Cu)(3) = 117(3) x 10-4 cm(-1), which show the low symmetry of the copper site and are assumed to be the "molecular" parameters. The EPR results are discussed in terms of the structure of Zn(L-asp) and the electronic properties of the copper ions. We analyze the temperature dependence of the g and copper hyper fine values considering a dynamic Jahn-Teller effect in terms of a potential surface warped as a consequence of tetragonal and orthorhombic lattice strains. Above 100 K, the behavior is explained in terms of the Boltzmann equilibrium between two isomer configurations distorted along the Cu-water oxygen bond (Silver-Getz model). The energy splitting between the wells are delta(23) = 235(5) cm(-1) and delta(13) .congruent to 440(50) cm(-1). Below 100 K, the data indicate the restricted validity of the SG model and suggest that the complex is localized in the deepest well.