. Longitudinal and transverse N-15 NMR relaxation times in human ubiquitin have been measured at 600-MHz H-1 frequency with a reproducibility of better than 1%. Two independent measurements of the N-15-{H-1} NOE indicate a random error of ca. 0.01, and no values were larger than the theoretical maximum. The relaxation data are incompatible with isotropic rotational diffusion but agree well with an axially symmetric rotational diffusion tensor with a diffusion anisotropy, D-parallel to/D-perpendicular to Of 1.17 There is no statistically significant further improvement in the fit between the experimental data and those predicted by a fully asymmetric diffusion tensor, confirming that the rotational diffusion tensor of ubiquitin is axially symmetric within experimental uncertainty. The relative ratio of the principal components of the inertia tensor calculated from the X-ray structure is 1.00:0.90:0.64, and the axis with the smallest inertia component makes an angle of 11 degrees with the unique axis of the experimentally determined diffusion tenser. Hydrodynamic calculations agree well with experimental results, provided half a shell of bound water is included and flexibility of the C-terminal residues is accounted for either by omitting them from the calculations or by using conformations for these residues obtained from a Langevin dynamics simulation.