  Polarized beam neutron scattering measurements on a highly perfect crystal of
${\rm YBa_2Cu_3O_{6.6}}$ show a distinct magnetic transition with an onset at
about 235K, the temperature expected for the pseudogap transition. The moment
is found to be about 0.1 $\mu_B$ for each sublattice and have a correlation
length of at least 75 \AA. We found the critical exponent for the magnetic
neutron intensity to be 2$\beta$ =0.37$\pm$ 0.12. This is the proper range for
the class of transition that has no specific heat divergence possibly
explaining why none is found at the pseudogap transition.
