Development of new ternary superconductors has led to materials which also show a strong tendency toward magnetic order when one of the constituent elements is a rare earth. Powder neutron diffraction data on superconducting (Ts∼6 K) samples of ErMo6Se8 taken in the temperature range 0.05-2.0 K show that magnetic Bragg peaks develop at T M=1.1 K, in agreement with specific heat studies. However, it is not possible to index these new reflections using a simple antiferromagnetic unit cell based on the ErMo6Se8 lattice. It is also not possible to index the magnetic reflections based on a single modulation vector, or with a modulation vector along a high symmetry direction including higher order harmonics. Thus either the long range magnetic order corresponds to a more complicated magnetic structure in the ErMo6Se8 lattice, or at least some of the peaks develop in impurity phases. These results are compared with the recent neutron data on the reentrant superconductors HoMo 6Se8 and ErRh4B4, in which the development of ferromagnetic order is clearly shown to be responsible for quenching the superconductivity.