Neutron diffraction measurements were made on erbium single crystals in the temperature range 298°–4.2°K.The material is antiferromagnetic below 80°K and ferromagnetic below 20°K. In the antiferromagneticregion, the magnetic scattering consists of satellite reflections corresponding to a modulation of themagnetic scattering amplitude along thecaxis. The spacing and intensity distribution of these satellitesshow two distinct subregions of antiferromagnetism. In the upper region, between 80° and 52°K, the datasuggest a sinusoidal modulation of the magnitude of thec-axis component of magnetic moment with aperiod of3.5 c0. Between 52° and 20°K the wavelength of the modulation varies continuously from3.5° c0to4.0 c0. In addition, there is a squaring up of the modulation and a simultaneous ordering of the componentof the moment normal to thecaxis. Below 20°K the material is basically ferromagnetic with a momentof 7.2 &mgr;B directed parallel to thecaxis.