Neutron diffraction experiments have been used to investigate the structural and magnetic ordering in the moderately disordered heavy fermion compound UPd_1.85Sn. The material crystallizes in the fully ordered cubic Heusler lattice of Fm3m symmetry. Below the antiferromagnetic transition temperature T_N=25.7 K magnetic Bragg scattering has been observed. The magnetic spectrum can be accounted for by an antiferromagnetic wave vector \vec {k} = [0 0 0],  i.e., the magnetic and crystallographic unit cells are identical. The magnetic moment is evaluated to \mu _ord = 0.97(9) \mu _B.

PACS numbers: 75.25.+z, 75.30.Mb, 75.50.Ee