  The antiferromagnetic structure of Yb$_{3}$Ga$_{5}$O$_{12}$ is identified by
neutron diffraction experiments below the previously-known transition at
$T_{\lambda}=54$ mK. The magnetic propagation vector is found to be ${\bf
k}=(1/2, 1/2, 0)$, an unusual wave-vector in the garnet structure. The
associated complex magnetic structure highlights the role of exchange
interactions in a nearly isotropic system dominated by dipolar interactions and
finds echos with exotic structures theoretically proposed. Reduced values of
the ordered moments may indicate significant quantum fluctuations in this
effective spin-1/2 geometrically frustrated magnet.
