  We measure the self-diffusion of colloidal rod-like virus {\it fd} in an
isotropic and nematic phase. A low volume fraction of viruses are labelled with
a fluorescent dye and dissolved in a background of unlabelled rods. The
trajectories of individual rods are visualized using fluorescence microscopy
from which the diffusion constant is extracted. The diffusion parallel
($D_{\parallel}$) and perpendicular ($D_{\perp}$) to the nematic director is
measured. The ratio ($D_{\parallel}/D_{\perp}$) increases monotonically with
increasing virus concentration. Crossing the isotropic-nematic phase boundary
results in increase of $D_{\parallel}$ and decrease of $D_{\perp}$ when
compared to the diffusion in the isotropic phase ($D_{iso}$).
