In this present study we report millimeter and submillimeter high-field CW EPR spectra of P700(.+), the primary electron donor in photosystem I. The data shown are the first well-resolved chlorophyll radical spectra observed without prior deuteration. The signal was generated from isolated plant photosystem I (in both digitonin and Triton-X100 preparations) by photooxidation. At 325 GHz and higher frequencies, the spectrum was resolvable into the three principal components of its g-matrix. They were obtained to high accuracy by spectral simulation and were found to be g(xx) = 2.003 17(+/-7 x 10(-5)), g(yy) = 2.002 64(+/-7 x 10(-5)), g(zz) = 2.002 26(+/-7 x 10(-5)) at 40 K and g(xx) 2.003 07(+/-7 x 10(-5)), g(yy) = 2.002 60(+/-7 x 10(-5)), g(zz) = 2.002 26(+/-7 x 10(-5)) at 200 K. These values indicate a temperature dependence of the g-values in P700(+) that was measured over a temperature range of 5-260 K using both preparations. The results showed that g(xx) changed with temperature while g(yy) and g(zz) remain constant within the error margin, suggesting that the P700(.+) radical becomes less anisotropic with increasing temperature.