  We use inelastic neutron scattering to demonstrate that at low temperatures,
the low frequency magnetic fluctuations in YBa_2Cu_3O_{6.6} ($T_c=62.7$ K) are
incommensurate, being found at positions displaced by $\pm\delta$ ($0.057\pm
0.006$ r.l.u.) along the $[\pi,\pi]$ direction from the wave vector $(\pi,\pi)$
associated with the antiferromagnetic order of the parent insulator,
YBa_2Cu_3O_{6}. The dynamical susceptibility $\chi''(q,\omega)$ at the
incommensurate positions increases on cooling below $T_c$, accompanied by a
suppression of magnetic fluctuations at the commensurate points.
