Magnetic ordering was investigated by neutron scattering in an YBa2Cu3
O6+x single crystal. We observed antiferromagnetic ordering (AFI) (T-N
= 410 K for x = 0.1 and T-N = 368 K for x = 0.18), but found no evide
nce for a reordering down to 2 K (AFII). The magnetic structure factor
s are presented to emphasize the anisotropic character of the form fac
tor.