  We report the magnetoresistance of a charge-density wave (CDW) in $o$-TaS$_3$
whiskers at 4.2 K under a magnetic field up to 5.2 T. An anisotropic negative
magnetoresistance is found in the nonlinear regime of current-voltage
characteristics. The angle dependence of the magnetoresistance, studied by
rotating the magnetic field upon the $c$-axis, exhibited a two-fold symmetry.
The magnetoresistance amplitude exhibits maxima when the field is parallel to
the $a$-axis, whereas it vanishes to the $b$-axis. The observed anisotropy may
come from difference in interchain coupling of adjacent CDWs along the $a$- and
$b$-axes. Comparison of the anisotropy to the scanning tunneling microscope
image of CDWs allows us to provide a simple picture to explain the
magnetoresistance in terms of delocalization of quantum interference of CDWs
extending over the $b$-$c$ plane.
