The flavin cofactor of Escherichia coli DNA photolyase in its neutral radical form, FADH(.), was investigated by high-frequency/high-field continuous-wave electron paramagnetic resonance at 360 GHz. The data presented are the first flavin radical spectra where the full rhombic symmetry of the g-tensor is resolved. A fit of the spectrum yields accurate principal values of g, which show only a small anisotropy: g(X) = 2.004 31(5), g(Y) = 2.003 60(5) and g(Z) = 2.002 17(7). The hyperfine splitting observed in the g(Y) region could be assigned to a hyperfine tensor component of the H(5) proton in the 7,8-dimethyl isoalloxazine moiety of FADH(.). From a comparison of this effective hyperfine coupling with the principal value obtained from pulsed (Davies) electron-nuclear double resonance, the orientation of the g-tensor principal axes with respect to the H(5) hyperfine principal axes could be derived. Remaining ambiguities in the sign of the angle between the principal axes of g and the molecular axes are discussed by taking into account results from g-tensor calculations using density functional theory and semiempirical AM1 based methods.