  We have observed a Fermi-surface (FS) induced lattice modulation in a YBCO
superconductor with a wavevector along CuO chains, {\it i.e.} ${\bf
q}_1$=(0,$\delta$,0). The value of $\delta\sim0.21$ is twice the Fermi
wavevector ($2{\bf k}_F$) along {\bf b*} connecting nearly nested FS `ridges'.
The ${\bf q}_1$ modulation exists only within O-vacancy-ordered islands
(characterized by ${\bf q}_0$=$(\frac14,0,0))$ and persists well above and
below $T_c$. Our results are consistent with the presence of a FS-induced
charge-density wave.
