We report a computer simulation study of the electron paramagnetic resonance (EPR) spectral line shape of-the iron-molybdenum cofactor of nitrogenase. The unusually broad and asymmetric line shape of the EPR spectrum can be interpreted in terms of a distribution of zero-field splitting parameters called D-strain. The best fit simulations were computed using D = 2.5 cm(-1) and E = 0.317 cm(-1) and distributions in D and E approximated by Gaussians of half-widths 0.446 cm(-1) and 0.108 cm(-1), respectively. The value of D estimated in the present work is smaller than previous estimates by others but consistent with the temperature dependence of the EPR spectrum. The large D-strain is most likely caused by an ensemble of nearly isoenergetic conformational states and should not be considered as being indicative of chemical inhomogeneity.