High-field EPR experiments at 12 T/330 GHz were performed on chlorophyll a radical cations in methylene chloride at low temperatures. Using fully deuterated chlorophyll it was possible to obtain the principal components of the rhombic g-tensor as g(alpha) = 2.00329 +/- (5 x 10(-5)), g(beta) = 2.00275 +/- (8 x 10(-5)), and g(gamma) = 2.00220 +/- (8 x 10(-5)). Protonated chlorophyll radicals did not give enough spectral resolution to yield the g-anisotropy from the EPR spectrum. This was true even at higher field/frequency combinations up to 24 T/670 GHz, Semiempirical calculations were performed using the INDO/S method (ZINDO) which yielded good agreement with the experimental data.