We report a measurement of an axially symmetric g tensor for the substitutional nitrogen center in type-IIa synthetic isopure C12 diamond. Because the nitrogen concentration of the diamond studied is exceptionally low the electron-paramagnetic-resonance linewidth is sufficiently narrow to reveal the g-tensor anisotropy. Studies of the angular dependence reveal  symmetry and an axial g tensor in which gperp-g=0.0001±0.000 03. From the narrow linewidth, we observe that the deviation from the  symmetry exhibited by the nitrogen center is less than 0.5°. © 1994 The American Physical Society.