The magnetic structures of a rare earth superlattice in an applied magnetic field have been determined using neutron scattering techniques. The sample was grown using the LaMBE facility in Oxford and the periodic unit consisted of 41 planes of holmium and 16 planes of yttrium. The results showed four different types of magnetic phase corresponding to the helix, helifan, fan and ferromagnetic phases observed when a magnetic field is applied in the basal plane of bulk holmium. In the superlattice the correlation length was very long for the ferromagnetic phase, about 10 superlattice periods for the helix, about 4 superlattice periods for the fan and there was little, if any, correlation between the superlattice blocks for the helifan phase. The structures depended in detail upon the history in field and temperature as the configuration was approached. The most striking aspect of the result is that the helical phase is stable to higher fields in the superlattice than in the bulk. It is argued that this is because the conduction electron susceptibility in yttrium favours a modulated structure more strongly than in holmium and that the propagation of conduction electrons throughout the superlattice transfers this enhancement to the holmium blocks.