Single crystals of KOH-doped ice Ih, transformed into the low-temperature ordered phase known as ice XI, have been studied by neutron diffraction. High-resolution measurements show a splitting of the 00l diffraction peaks into an ice XI and a residual ice Lh component, with about 37% transformation to ice XI, New diffraction peaks from the ice XI have been measured. These are consistent with a ferroelectrically ordered structure with space group Cmc2(1).