  Measurements of the nonlinear flux-flow resistivity $\rho$ and the critical
vortex velocity $\rm v^*_\phi$ at high voltage bias close to the instability
regime predicted by Larkin and Ovchinnikov \cite{LO} are reported along the
node and antinode directions of the d-wave order parameter in the \textit{a-b}
plane of epitaxial $YBa_2Cu_3O_{7-\delta}$ films. In this pinning-free regime,
$\rho$ and $\rm v^*_\phi$ are found to be anisotropic with values in the node
direction larger on average by 10% than in the antinode direction. The
anisotropy of $\rho$ is almost independent of temperature and field. We
attribute the observed results to the anisotropic quasiparticle distribution on
the Fermi surface of $YBa_2Cu_3O_{7-\delta}$.
