The importance of large-scale magnetic fields in H I rotation curves of spiral galaxies is investigated. In contrast to other papers that try to render dark matter unnecessary, we assume that galaxies are embedded within a modified isothermal dark halo. The NGC 1560 magnetic field is derived from a simple prescription suggested recently by Sanchez-Salcedo for large luminous spirals. The detailed shape of the observed rotation curve fits reasonably well with a magnetic field strength ~1muG. This magnetic fit is as good as that for modified Newtonian dynamics (MOND). We conclude that prominent structure in the rotation curves of dwarf galaxies is compatible with dominant dark haloes.