  We report novel features in the in-plane magnetoresistance (MR) of heavily
underdoped YBa_2Cu_3O_{6+x}, which unveil a developed ``charged stripe''
structure in this system. One of the striking features is an anisotropy of the
MR with a "d-wave" symmetry upon rotating the magnetic field H within the ab
plane, which is caused by the rotation of the stripes with the external field.
With decreasing temperature, a hysteresis shows up below ~20 K in the MR curve
as a function of H and finally below 10 K the magnetic-field application
produces a persistent change in the resistivity. This "memory effect" is caused
by the freezing of the directionally-ordered stripes.
